Ditched by My Sister

Heres the deal the first day in college I met this girl Jess. I instantly could tell that my roommate had a thing for her, so I knew she was off limits. Plus for some reason I didn't find her attractive at all. Well my roommate isn't one to party because he can get rowdy and accidentally insult people. So, it may be better altogether because I really hate having to fight for other people's stupidity. Anyways Jess and I like to go out and have a good time, but heres the catch. We inform everyone that she is my sister so nobody messes with her because she already has a man. Jess and I had been looking forward to a shindig all week, and when it finally came we made an appearance and began having a good time, unfortunately she felt the need to head home after being there for only like 30 min. She did this because my roommate and her apparently got into a fight about nothing and she decided it would be better to leave. I just got kind of frustrated because we have been a pretty dynamic duo at these shindigs and now it's all ruined. I'm not sure if I should ever go out with her again because if she's ok with leaving her partner in crime once its more than likely she will do it again.