My boyfriends sister flirts with him! what should i do?

So, I live with my boyfriend, our daughter, and his family. This is all fine and dandy with the exception of one oozing sore on the situation, his sister. First of all, my boyfriend and I are twenty and his sister is a few months shy of 18. I have always been a target for her simply because I am a woman. She competes like crazy with any woman and believes there is always a "better" woman. This is exhausting for me as I do not do the whole "competition" thing. For two years I have tried to ignore her and carry on doing my thing. But then everything changed. The problem began when her boyfriend moved in. She started becoming extremely jealous. Swearing up and down that he was looking at me all the time. We would all be hanging out and if he even said one word to me she would go ballistic. Then she began trying to get back at me. She started flirting with my boyfriend, her brother. She would touch him on the upper thigh or bend over in front of him wearing booty-shorts. I found this extremely frustrating because I was going out of my way to avoid doing anything provocative around her boyfriend! She has always been very inappropriate towards her brothers. From my view it looked like she just wanted sexual attention from EVERY male in her vicinity, including her brothers. I found this a little weird. Then I found out a tidbit of disturbing information. She was "raped" by her eldest brother. I put quotes on that because he claims he was drunk and she actually initiated the sex. This is debatable as I was not there, I cannot say for sure, but the way she acts does leave "rape" a little questionable. Now I worry that she would try to sleep with my boyfriend! I just don't know how far she would take this flirting bullshit. I approached my boyfriend on the subject, asked why he lets her touch him like that. He said there's nothing he can do about it. If he says anything to her about it she goes straight to their mother (who also lives with us). His mother will turn it around saying HE is perverted for thinking of it like that. He does not like talking about all this as it is very uncomfortable for him and he feels bad for his little sister. When his mother turned it on him I realized why his sister is that way. She's always gotten away with it! Their mother condones her daughters bad behavior and turns it on the boys if they stand up and say anything. It's almost as if these boys are victims of their little sisters sexual harassment! I realize a good solution would be to move out but we are a little financially strapped right now with the arrival of our new baby girl. I need advice! How do I deal with this situation? How do I handle this nasty little girl and keep her off my man!?