During college, my then-best friend called me and told me to drive to a hotel he was staying at with his family for winter break. I started the three-hour drive, just because I had nothing else to do.
I get to the hotel and found everyone at the pool...my friend and his sister (only 10.5 months apart, no joke, so we were all in college), his parents, and a family "uncle" figure. I walked alone to their hotel room, put on my swimsuit, then walked back to the pool. His sister's tiny bikini top on her enormous boobs was slipping a little, I was caught staring several times, and her mom kept telling her to adjust her top.
After swimming, my friend, his sister, and I walked back to the hotel room. While waiting for my friend to dig out the room key, I thought that maybe I would be daring and change clothes in front of his sister. It was worth a shot. I started getting hard in the hallway thinking about it.
We get into the room, and within 10 seconds I have my swimsuit off and I am totally naked. My friend said, "Gnome, my sister is here!" I said, "so?" and dried off with a towel while she was staring, and my friend took his swimsuit off, too (they shared a room at home their entire lives and and seen each other a million times, so this wasn't anything of note).
His sister said, "I would let you see me change but my mom would get mad if she finds out," (even though she was like 19-20yo), then she walked into the bathroom to change, which I thought was too bad. The bathroom had a sliding door, but she didn't close it all the way, so I moved to the side to see her. My friend said, "Gnome is trying to watch you!" and I heard her giggle. She leaned her head back to look out the door and stared at my totally hard cock now.
She untied her top, but she was juuuuuust inside the door enough that I just saw sideboob. And there was zero sag, even though she was at least a DD. It was unreal.
But then she closed the door the rest of the way to take her bottoms off.
She came out in a loooong t shirt to sleep in, clearly no bra, and possibly no panties, but I couldn't tell.
I was still naked because she was really fast and I literally couldn't find my own clothes. Their hotel room was a mess and I found them under the top of a suitcase, and I got dressed while she stared at me.
Then I ended up driving back to my college (why didn't I stay the night???? I don't remember why), and I remember jerking off to that evening for years to come.