All through lunch at the mall, Liz could hardly concentrate and picked at her food as her friends chattered away. She was trying to process all the troubling and exciting emotions coursing through her 18-year-old brain.

Between her and her twin brother Luke, Liz had been the dominant sibling for as long as she could remember. She had always tried to act like the big sister and enjoyed it immensely whenever he got treated as less mature than her. This was especially true when it came to sick times. Even though Liz had switched to oral temperatures before puberty, she knew that their mother continued to use the baby thermometer and take Luke’s temperature rectally. The thought of her brother being treated like a little boy always brought a strange titillation to her, and she took every opportunity to take care of him and try to get a glimpse of the rectal thermometer sticking out from his bottom when Mom checked his temperature. It was especially satisfying to go the doctor’s office together, when Luke would inevitably have to lower his underpants for his temperature, while Liz was allowed to have hers taken in her mouth. The week before, when Mom was travelling and Luke caught a strep infection, Liz had conspired with their next-door neighbor Katie (who had become Luke’s de facto nurse/babysitter) to take her brother’s temperature herself for the very first time. That experience ignited a sense of power and control that made her slightly giddy.

But with Ben, her emotions were on a whole different level. Their next-door neighbor (and Katie’s younger brother) had been Luke’s best friend since childhood. He had grown into a rather good-looking young man and, now at 18, sported a fair bit more body hair than Luke. Liz occasionally found herself fantasizing about him, especially his tight butt (which she had unfortunately only ever seen through a swimsuit), when she masturbated. But now, the knowledge that his sister Katie, who was two years older than the three of them, still took his temperature rectally when he was sick (and had done so just that morning!) sent those fantasies into overdrive. Her desire to take care of him was now coursing through her body and brain. When Katie had asked her to “check in on him” after lunch to see if his fever was spiking, she practically had an orgasm in anticipation.

Liz wanted to have a maximum effect on her “patient” when she took charge, so she stopped by her home to prepare before heading next door. Mom had left on another business trip, and her brother was dozing on the couch in the den in front of the television; the poor guy was still not 100% well and would probably end up again at Village Pediatrics, a thought that sent more tingles down Liz’s spine. She went to her room and slipped off her street clothes as she pondered what to wear. Seeing herself naked in the mirror gave her some satisfaction: although she found her hips a bit on the wide side, her body was fit, and her creamy-white b-cup breasts were soft and perky. She had noticed the previous summer how Katie, eschewing social norms, had let her body hair grow. Liz had decided to surreptitiously follow the example of her older neighbor. Now, seeing the tufts of hair under her arms and the thick, untrimmed triangle of dark blonde pubic hair gave her a renewed sense of power and somehow made the upcoming visit to Ben all the more exciting. She donned a pair of gym shorts and a light, loose fitting tank top, through which her large areolae were just barely perceptible.

All was calm next door at the Richards’ house as she let herself quietly in the front door. Mr. and Mrs. Richards were away for the week at their lake home, and Katie wouldn’t be back from her job interview for at least a few hours, so Liz knew that Ben would be alone. She made her way down the main hall and peeked in the door to his bedroom. She got a little frisson seeing him curled up asleep in his bed. He definitely looked paler than usual, which got her slightly concerned. But at the same time, seeing him looking so vulnerable amplified her state of excitement.

Unlike Luke, Ben still slept on a boyish “captain’s bed”, with the mattress supported on a solid platform of drawers about three feet above the floor. That would make it impractical for her to place him over her lap to take his temperature, she noted with some disappointment. On the other hand, the set-up reminded her strangely of the large examination tables at Village Pediatrics. She thought back to the joint appointment the previous week when she got her tetanus booster in her arm and managed to stay for Luke’s exam. She watched him get his temperature taken rectally twice: once by the nurse practitioner Jill Smith at the beginning of the exam, then later after his antibiotics shot (in his bottom, of course!), by their former high school classmate Debra Hobbs, who was doing a pediatrics internship before starting nursing school. Luke had been beet-red the whole time as he lay on the high table with his bare bottom exposed just inches from the faces of his caregivers. At the time, Liz remembered feeling somewhat envious of Debra’s power over someone their own age. But she realised now that she might be able to replicate the scenario here in Ben’s own bedroom, complete with a remarkably similar “exam table”.

Rather than wake Ben right away, she ventured into the master bathroom. Katie had indeed left out the thermometer as promised. Also on the counter were a jar of Vaseline and a notepad with a chart like the one Katie had made for Luke the week before. The sole entry was 101.2 at 9:30 that morning, and Liz shivered slightly from pleasure when she saw the word “rectal” written as the method used. At the sight of all that, Liz felt her nipples press against the fabric of her tank top, as well as the beginning of moisture between her legs.

Liz next went to the kitchen and poured a glass of water. She added several ice cubes from the freezer before heading back to Ben’s room to begin her ministrations.

Ben stirred as she approached the side of his bed and set the glass on the nightstand.

“Liz? What are you doing here?” he asked in a groggy and somewhat hoarse voice, trying to prop himself up.

“Your sister is out for a few hours, but she’s really worried about you. She asked me to pop over to check in on you,” Liz answered, trying to muster a big-sister tone even though they were the same age. “How are you feeling?” she asked, gently pushing him back onto his pillow, a move that he didn’t resist.

“Not great, actually,” Ben responded still groggily. “It started as an earache last night, but now I feel like my throat is on fire. But at least I’m not coughing anymore.”

Liz reached over and placed her hand on his forehead. As she did so, she leaned forward slightly, and the front of her tank top fell away a few inches from her body, exposing the tops of her breasts and her nipples to Ben’s glassy gaze. Liz felt another twinge of excitement as she realised that he was very warm to the touch, but she did her best to project a maternal look of concern.

“Hmmm, you’re burning up. I hope you didn’t catch Luke’s strep. Have you been feeling any chills?”

Ben nodded feebly in response to the question.

“OK. Well, try to drink this,” she said as she handed him the glass of cold water. “Hopefully, it will soothe your throat a bit. Meanwhile, I’ll see what I can find.”

With that, she headed out the door. Looking back, she got a sense of satisfaction as Ben sat up briefly and downed several sips of the ice-cold water; as if she needed another argument, at least now she knew that it would be impossible for her to get an accurate oral temperature reading for the next 45 minutes. By the time she reached the master bathroom again, she was tingling all over and could feel more dampness in her crotch. She paused a few moments to steady herself, then picked up the thermometer and Vaseline and returned to Ben’s bedroom.

Ben stared at her from his captain’s bed as Liz placed the Vaseline on the nightstand, removed the stubby-tipped thermometer from its case and started shaking it down.

“Um, Liz? What are you doing?” asked Ben with some trepidation in his voice.

“I need to take your temperature,” she responded matter-of-factly, as if it were the most normal thing in the world. She then returned her attention to the thermometer. Once she had confirmed that the column of mercury was below 96, she opened the jar of Vaseline and dipped the tip of the instrument in it.

“Why did you get the baby thermometer?” Ben’s trepidation seemed to have turned to panic.

Liz turned toward him holding up the now-lubricated thermometer where he could plainly see it.

“I’m sorry, Ben, but I need to get an accurate temperature. Between your sore throat and the fact that you just drank cold water, there’s no way to get a reliable oral reading,” Liz explained in as calm a voice as she could muster, not wanting to betray her own emotions. “Besides,” she continued with the faintest hint of a smile, “I know that Katie took it in your bottom this morning, so we need to get a valid comparison, in order to see how your fever is evolving and decide whether to take you to the doctor.”

Ben’s previously pale face suddenly turned bright scarlet as he realised that Katie had told Liz how she had taken his temperature that morning. He seemed crushed with embarrassment and wondered whether his sister had also mentioned that it was in fact not the first time that she had used the rectal thermometer on him. To aggravate things, Liz’s firm tone left no doubt about who was in charge. He felt beaten already.

Sensing that she was gaining the upper hand, Liz went on. “You know, strep can be really serious, so we shouldn’t take any chances. Luke still hasn’t fully recovered, and it looks like we may have to take him back to doctor again thanks to you boys’ irresponsible swimming outing to the creek.”

She concluded on a note that was softer but that further underscored her now obvious authority. “Don’t worry. I’ll be gentle with you.”

With that, she put the thermometer down on a tissue, reached over and slowly pulled the bedsheets down to Ben’s feet. He was wearing just a pair of white cotton briefs, and there was a visible bulge where his penis was pushing against the fabric. Ben seemed to be riveted in place, afraid to move, as Liz stared at his crotch with a smug smile on her face. She finally broke the silence by playfully asking, “So, are you going to turn over for me before or after I pull down your underpants?”

Ben didn’t need to be asked twice. He quickly turned over, happy to hide his embarrassing erection. Liz took a moment to admire her power over him in this vulnerable moment. She grabbed the waistband and instructed him to raise his hips as she lowered the briefs to his knees. She was temporarily mesmerised by what she saw: Ben’s buttocks were pale and firm, with a bit of black hair that became slightly thicker closer to his crack. She was afraid she might just have an orgasm right then and there but managed to keep control of her emotions.

Ben’s head was turned to the right so he could see her as she reached over, once again partially exposing her breasts just inches from his face, and picked up the lubricated thermometer. She then turned and faced him fully as he lay prone on the elevated bed. She was again reminded of the exam tables at Village Pediatrics and the memory of how both Jill and Debra had gently but thoroughly dominated her brother when he had been in the same position. Her eyes were now only about a foot from Ben’s beautiful, bare behind. She placed her right hand on his butt then slowly, almost sensually, slid it down, pushing her thumb and forefinger deeper into the glorious crack before spreading him wide open.

They both gasped simultaneously: Ben from the sudden rush of cold air on his most intimate body part, and Liz from the sight of Ben’s pink, crinkly anus, which had suddenly appeared among the hair. She also perceived a slight musky odor emanating from him. Coupled with the sight of the back of his scrotum, that only served to heighten her arousal and the growing moisture between her legs.

Trembling slightly from excitement, Liz placed the Vaseline-coated tip of the thermometer against Ben’s anus, then deftly pushed it inside, causing him to let out a soft moan. She intently watched the excess Vaseline bunch up around the stem of the instrument as it slipped past his sphincter and plunged deep inside. Once it was inserted about halfway, she reluctantly released his cheeks and let them close around the exposed stem. She cupped her hand on his butt and held the thermometer firmly between her fingers, the way she had seen in baby-care books in the bookstore.

“There, now, that’s not so bad, is it?” Liz asked in a light, slightly mocking tone, revelling in the power that she now had over her vulnerable man-boy patient. She recalled Katie’s observation that Ben had gone completely passive once the thermometer was inside him and noticed the same reaction now.

For his part, Ben was so focused on trying not to ejaculate that he couldn’t answer more than a slight croak. The combination of Liz’s newfound big-sisterlike assertiveness, which he found weirdly attractive, and the feeling of the cold thermometer sliding deep into his rectum was almost too much to bear. His erect penis pressed against the bedsheet, and he felt like he was on the verge of exploding. It didn’t help that Liz occasionally respread his buttocks and pushed the instrument to the side as she checked the progress of the mercury

As they waited, Liz decided to probe for more information.

“So, Ben, do you get sick often?” she asked innocently, pretending to make small talk.

The question caught him by surprise, and he had to think about it a moment.

“Um…. Sometimes…. Not very often, I guess,” was all he could muster.

“Katie told me she usually has to take care of you when you do, at least when she’s home from college,” Liz continued, trying not to divulge the true aim of her line of questioning. “You’re lucky to have a big sister like that!”

Ben thought to himself that “lucky” maybe wasn’t the best adjective, so he just grunted an unintelligible response.

Determined to get to the heart of the matter, Liz then zoomed in with her zinger: “And does your mom also still use the baby thermometer on you?”

The young man was slightly flabbergasted by the question, not only because it was so personal, but because it implied that Katie had indeed shared a lot more information than he would have preferred.

“No, of course not!” Ben quickly replied, pretending to be indignant. But then, realising that Liz maybe had more information and not wanting to be caught in a lie, added: “At least not since I changed to Dr. Jones.” The fact that he hadn’t actually been sick since changing to an adult doctor made it a convenient escape.

“Oh, when was that?” Liz continued sweetly.

“Um… about six months ago,” he was forced to concede, his faint bravado quickly melting away.

Liz just smiled. Even though she already knew the answer, the idea of her gorgeous next-door neighbor admitting that he still submitted to rectal temperatures by both his mother and sister was incredibly arousing. She felt more flutters, not only from the conversation, but also from the sight of the thermometer protruding prominently from between his tight cheeks.

Finally, she broke the awkward silence.

“OK, five minutes are up. Let’s check this.”

Liz again spread Ben’s cheeks and slowly removed the thermometer. She held him open for just a bit longer than necessary as she discreetly admired his now slick rosebud before reluctantly releasing his cheeks. She wiped the thermometer with a tissue and held it up to read it as Ben watched expectantly, his bare bottom still exposed.

“Hmm, 102.3,” she said with a frown that masked her internal exhilaration. “Katie asked me to call Dr. Jones if your fever worsens and try to set up an appointment for later today. Why don’t you rest while I do that?”

She helped him to raise his underwear back up, stealing a brief but satisfying view of his engorged penis as he raised his hips to let the waistband pass. She then pulled the bedcover back up before he turned over, a gesture that gave him at least a modicum of privacy.

Liz returned to the master bathroom. After rinsing the thermometer in cold water and replacing it in its case, she noted Ben’s temperature on the chart, giddily repeating “rectal” as the method used. She then went to the phone to call the doctor’s office.

From his bedroom, Ben could hear only snippets of her conversation, which lasted about a minute. He then heard her make another, longer call. Apart from “102.3”, he couldn’t really make out what she was saying but was too tired to think about it. Liz soon reappeared, her loose breasts bouncing freely under her tank top. She handed him a couple of pills and the glass of water.

“Here, take these, Kiddo. They’ll help get the fever down.”

Ben slightly resented someone his own age calling him “Kiddo” but was nonetheless glad to accept her care. He downed the pills with some difficulty, while struggling to keep his eyes off her obviously erect nipples.

“Dr. Jones’s office is fully booked today and tomorrow,” Liz went on. “Fortunately, I was able to get you in to one of the last appointments of the day with the new nurse practitioner at Village Pediatrics. I know you’re too old, but Katie told me it would be a good back-up plan, and anyway much better than the emergency room! The appointment is at 4:30; I’ll drive you if your sister isn’t back by then.”

As she said that, Ben detected a rather strange, faraway look on her face. Despite having known her all these years, he wasn’t sure how to interpret it. It was also bizarre to think he was going back to a pediatrian’s office despite being an adult now. But he was too tired to think much of it and finally slipped off into a fitful nap.