In our family corporal punishment were never used, so during my entire childhood and until that day I'd never been spanked. At the time I was 13 yo and I was spending the summer holidays in our seaside cottage with my two older sisters, Rebecca and Antonia, aged respectively 19 and 17 years. Our parents joined us every weekend. That afternoon, back from the beach, my sisters have stopped at the grocery store to do some shopping, while I had returned home alone. I'd gone into the bathroom to take a shower and I'd just removed the swimsuit shorts with my back turned to the door, when I heard someone enter the room. Thinking it was one of my sisters, I've not even turned the head and went on take off my T-shirt. "Wow! What a nice little butt!" exclaimed behind me a vaguely known voice. I turned abruptly with a gasp and found myself face to face with a beautiful girl wearing nothing but a beach wrap over her bikini and a pair of flip-flops. Immediately I felt myself die from shame: that girl was the eighteen-year daughter of our neighbors, for which I secretly felt an overwhelming youthful infatuation, even though she had never deigned to speak to me until then. "What ... what do you want? Who let you enter here?" I succeeded to stammer, while, red as a tomato, I was trying desperately to cover with the hands my private parts. "I've come to get a book on loan from Antonia." she answered in an indifferent tone of voice, without any respect for my evident embarrassment "The door was open, so I entered." "My sisters are out and there's only me here." I replied, hoping she would go away in a hurry. "Well, if so I'll come back another time ..." she said, continuing to look at me with impudent insistence and without showing any signs of leaving. Now, at the peak of frustration and exasperation, I heard myself scream: "So what? What the f++k are you still waiting for?" "Oops, now you're in trouble young boy! You must not talk dirty in the ladies' presence. I'm sorry, but before getting out of here I must teach you good manners." "What do you mean?" I asked hesitant. "I mean that now i'm going to spank this your bare ass!" So saying, she grabbed me by an arm and, a moment later, she was sitting on an edge of the bathtub and I was lying face down on his lap. I could have groped to resist, despite her athletic physique as skilled swimmer, but I'd been caught off guard and now, while she was already spanking my buttocks, I felt so excited that, instead of rebelling, I wished her to continue because the enjoyment in feeling my penis and my balls rubbing between her soft nude thighs far exceeded the burning of my ass and the embarrassment of that situation. "How dare you hit our brother, ugly witch?" screamed my eldest sister Rebecca, dashing into the room with Antonia and blocking the raised arm of our neighbor. Both my sisters were just come back home and wanted me to tell them what had happened. After being informed, Rebecca turned to the intruder with angry voice "Now you have only two options: either you agree to be spanked here and now, or we'll escort you home to inform your parents that you've harassed our thirteen-year brother. Choose quickly!" The girl tried in vain to justify herself and to find a way out but my sisters were inflexible, so she finally had to choose and, of course, chose the first option. "But at least not in front of the boy." she tried to obtain, pleading. "Quite the contrary, sweetie: in front of him and showing your naked butt." cut short Rebecca, who meantime had sent Antonia to take an old ping pong paddle and had gone to sit on the toilet seat "So get a move on, take off your bikini panties and lie face down on my lap!" Having lost all hope to avoid punishment, the hapless girl slipped off her panties and quickly assumed the required position, doing her best not to put in full view at least her pussy. When my sister raised up her beach wrap to mid-back, I was entranced by a pair of sculptural buttocks clenched in anxious anticipation of the impending pain. Then the first swat rang out, closely followed by several other. Soon she began to squirm wildly, screaming like crazy and begging to stop, but my sister went on unmoved to administer punishment. I can't say how long lasted, because that was the most arousing scene which I'd witnessed up to then and, of course, I didn't pay any attention to the passing of time. Finally, by dint of squirming and kicking, the poor girl let out a loud fart. "Well sweetie, the final whistle seems to have just been blown, so the match is over!" said Rebecca letting her go, and all of us siblings burst out laughing. She, sobbing, picked up her panties and ran out of the room without saying a word. Shortly after we heard the front door slamming. I will never forget that spanking.