Issue with the "Family"

Well my boyfriend's sister and i hav had our share of problems but they were always worked out. Whenever she needed something or advice i was always there. Now lately she has been having a friend around. I not knowing that girl was spending the night at her house. in the midst of it all she is trying to hook up my boyfriend and her friend. talk about a stab in the back. i was sooo mad. Thinking how in the world could she do that if she's my friend and when she calls me "sis"? I'm not worried over my boyfriend it's just the fact of his sister. So what do i do now?

Confront her about it. Simple.