You may not like this answer, but it's about facing reality and getting your priorities straight. If you've been with your bf for 10 years and you have your own place, this is similar to a marriage. In a marriage, your priority is your spouse, not your sister. You can be close to her but still acknowledge this. It also makes things easier.
So I'm on Team Boyfriend. Stop and think how you would feel if it was HIS family member living with you and causing all sorts of problems. I actually think it was nice of him to say yes to this, because most people wouldn't. There's a big difference between helping someone for a couple months vs letting them move in and disrupt your life.
I don't agree with the answer saying they both need to grow up. Your bf is in his own home and is free to act however he wants. It never bothered you until she showed up. But she doesn't have equal "standing" in your home, for the simple reason it's not her home. She's a guest.
So my suggestion is talk to her and find a way to explain she needs to back off. I lived with my sister and BIL for 5 months several years back, and I knew better than to act like she is. In fact, we ended up having a lot of fun. But I never forgot my place, and that's the problem here.