So I noticed it a tiny bit after I met him but I didn't think too much about it. I noticed that I occasionally caught him staring at her butt when she wasn't looking and he just played it off. But whenever she's on the phone with me or something he always goes into her room while she's changing and I see the first thing he looks at is her butt than she covers up and tells him to get out because she's naked and all he says is he's seen it 100 times before. Long story short I was in the kitchen of her house and no one was home except her brother but he was sleeping. We started to make out and then I had her jack me off while standing because there is a mirror in the kitchen that I looked at so I could see her butt jiggle. Anyways I noticed that her brother was hiding behind the couch staring directly at the butt. But this time he was recording it but I didn't tell her because I was almost finished and curious what he was doing. Than he left and stopped recording when she pulled her pants up. The next day he came to me and told me not to tell her that he recorded it otherwise he'll show it to his mom. And again later that day I saw him jacking off to the video zooming into her butt idk what to do.

TL;Dr My girlfriends brothers took a video of her butt and jacked off to it.