My older sister caught me masturbating when I was 12 and a half and she was about 16. I had been real horny all morning the day she caught me and kept getting frequent involuntary erections.
After we ate lunch our parents left to get more groceries, leaving my sister and I home alone. My sister then asked me if I wanted to ride my bike with her. I normally would've said yes but as I said I was really horny that day so I said maybe later and she said ok then left. I watched her ride her bike down the highway from my bedroom window, and since I now had the house to myself and thinking she would be gone for around 10 minutes, I closed my bedroom door, stripped completely naked, and started masturbating by humping my bed(since I didn't figure out I could masturbate by stroking my penis until I was 13).
My sister then returned home just a minute or 2 later and opened my bedroom door since I forgot to lock it. As she opened my door she asked "Are you sure you don't want to ride your bike with m-! Oh my god! What are you doing?!" and since my door was behind me my bare butt was facing right towards her. I then rolled off my bed sideways and landed on the floor with my legs bent at the knees and my hands on the floor like a frog. She then walked into my room, looked down at me, started laughing, and asked "Why is it twitching so fast?". I looked down and noticed that my penis was twitching like crazy to the beat of my racing heart. I then closed my legs together and yelled at her to get out as she ran out of my room laughing hysterically.
Then I got dressed as my erection faded away and looked out my bedroom window to see my sister riding her bike down the highway again. I then got on my bike and rode after her. I dont know what I was thinking(I guess I wanted to save face) but I rode my bike next to her as mine swayed side to side die to my legs shaking from the adrenaline rush i was still experiencing and said "I changed my mind I want to ride my bike with you now." as she said "Ok..." in a sarcastic tone before laughing. I then got embarrassed and blushed as i said "Nevermind!" then rode my bike back home before burying my face in my pillow in shame.
A few days later my sister had a sleepover with two of her friends who were pretty blonde haired, blue eyed twins. When they showed up I was in the kitchen as my mom was cooking dinner. My sister introduced us and told me to say hi. When I did they started giggling and I knew right then she had told them about catching me. As we ate dinner at the dining room table they kept looking up at me and smirking. My mother noticed this and smiled because she must have thought they thought I was cute(if she only knew!).
After we finished eating my sister and her friends went into the living room to watch a movie while I went into my bedroom to play a video game. They finished watching the movie a couple hours later and went into my sisters bedroom which was right next to mine, seperated by just a wall. I then heard them whispering and giggling so I paused my game and put my ear up to the wall to hear what they were saying. Then I heard one of them ask my sister "What did his penis look like?!" and my sister say "It was small!" before they all burst out laughing hysterically.
It felt like I had been stabbed in the heart as i felt simultaneously humiliated, enraged, and aroused. I then wondered if my mom and other relatives cared about penis size which made me feel sick to my stomach. Then I unpaused my game and played it for a couple more hours as I continued to hear them whisper and giggle before I went to bed. When I woke up the next morning I stayed in my room for a few hours while I pretended to be sleeping in until I looked out my window and finally saw them dive back home.
I dont know what they were expecting from me at that age. I would estimate that it was only 4 to 4.5 inches hard back then. Now its 6.5 inches hard but I'm still insecure about it thanks to my sister and how the media never shuts up about penis size.