My older brother brought his 20 year-old college girlfriend home for Labor Day weekend and she was a HOT blonde bombshell. I was super turned on just looking at her. So she was staying in our sisters room and left the door open and had her clothes strewn all over the room. I walked past the room and noticed this pair of silky aqua-colored panties that were inside out and obviously slightly soiled. I had no choice but to grab them and wank furiously in my room. I wanked twice in the same day. Putting them back after each time and going and getting them again. I made certain no cum got in them. The thought of pressing my cock where her pussy was and dried juices just turned me on. There was even a tiny dark ass stain and I wanked on that as well. The next day I snuck them out and was wanking again. Only instead of putting them back I decided to keep them in my pocket. A little later the two of them came home from somewhere and went upstairs. No one was home but me and they both went into my sisters room and shut the door. I was thinking “they are either fucking or napping” and for some reason.... instead of going somewhere private I just unzipped , took the silky panties out and started to jack right in the family room downstairs. While doing this she walked past me on way to kitchen- startled me and she winked and said “hope I’m not interrupting something”. Startled - I said “no I have jock itch” zipping up quick and hiding the panties... She got a glass or water and walked past me again on way upstairs to my brother and said “well have fun with that” winked and pointed at my crotch. I’m 100% certain she saw me wank, certain she saw her panties. and certain she was encouraging me. Damn I was sad when he broke up with her a year later.