TL;DR my sister had sex with my roommate and close friend in my bed while I was gone. How do I handle the breach of personal space, trust, and privacy on my friend's part?
Will try to keep this short but give enough context. I (24F) have a very rocky relationship with my sister (also 24F), who I'll call Julie. Basically, she has borderline personality disorder and doesn't go to therapy or do anything to help change her behaviors. She constantly splits (Google BPD splitting) on me and my parents and never tries to fix it. She has also, on multiple occasions, had sex with my friends and blown up my friendships/relationships by having sex with inappropriate people. Before this incident, we had been where we always were–a couple times a year I try to do something with her because I want a good relationship with my only sibling; she always ruins it and I'm upset until the next time when I try again.
The friend in question (27M) I will call Matt. He and I are (were?) very close friends. We're also roommates, each with our own rooms just down the hall from each other. We hang out all the time and he listens to me being hurt by Julie constantly. Just before Christmas, Julie was coming to stay for ONE night and Matt was having a couple of his friends also spend the night that night, so we decided it could be fun to all go out together. Matt had not met Julie in person, so I sat down with him and had the conversation that she WILL try to sleep with him because he is my friend and that is what she does. I advised him not to do it because 1) she has a boyfriend 2) she would not treat him well and 3) I really just want one friend who is truly my friend that has no entanglements with my fucked up sister. He said he totally understood and absolutely would not do it. I'm sure we all see where this is going.
That night comes around and Matt, Julie, Matt's two friends, and I all go to a club together. Things are going well until Julie starts her show and started hitting on Matt. I had also asked her not to do anything with him and she was appalled at the suggestion earlier in the night. She kept going up to dance on him and the first few times he stepped away, but finally did dance with her. I asked them to stop as it was making me uncomfortable and they did. Julie then decided she wanted to go home early, so I got in an Uber with her. She asked why I was annoyed and I said that I wished she wouldn't try to do anything with Matt. She split on me and lost her shit, so when we got back to my apartment, I made sure she had everything she needed and I left her on her way to bed in my room. I left the apartment for the night to wait out her splitting and also knew that Matt and his friends would be home soon in case she needed anything. Julie and I went to our parents' house the next day and she proceeded to absolutely blow up the holidays as usual (not really important).
Come to find out last night that that night, Julie and Matt had sex in my bed while I was gone. Matt came clean about it and said that he'd been feeling really guilty and knew he needed to tell me. He says this is what happened: he came home, his friends went to bed upstairs, and he came downstairs to find Julie in the hallway between our bedrooms. She came onto him pretty hard and he said no and went to his room. But, when he came out again to use the bathroom, she was still there and came on to him very hard again. He said yes and they had sex in my bed. They did not wash my sheets. I came home the next morning and napped in said sheets.
Here's the thing–they're both consenting adults and while I wish that Julie wouldn't sleep with all of my friends, I can't really control that. I just feel a little hurt that Matt slept with someone who is so problematic in my life. But, I have also seen how Julie comes onto people and I'm trying to understand that he was drunk and had a hard time saying no. What really bothers me is that they did it in my bed. Matt's room was available and 15 feet away. Julie does this shit as a power move and I'm just kind of done with her at this point, so I'm focusing on Matt because I just can't believe that my good friend would have sex in my bed with my sister without my permission when his room was RIGHT THERE. And as my roommate, what a massive breach of trust, privacy, and personal space.
I don't know if I'm overreacting and Matt has been a really good friend to me, but this just feels so incredibly over the line. I also don't really see how they ran into each other in the hall twice; it seems more likely that he knocked on my door to see her after the first time. How do I handle this?