Not to bring up the obvious, but didn't you just get back from a semester abroad where you were asking about staying with a guy you met in London in the Relationships area of GC in this thread
http://www.greekchat.com/gcforums/sh...threadid=50550
How can you be upset when you were dating others too?
I'm not excusing what your sister did, but if you were away and hooking up with other guys, what does it matter what she did with the guy you were seeing? Long distance relationships are ridiculous unless both parties are truly committed to it working, and if you are asking about staying with a guy you met in London then you weren't really that committed to the guy in the States.
Why not be mad at the guy? He was a willing participant. Yes, sisters should know better, but don't just blame her. People make mistakes, and I think that we all can forget that sometimes. So she moved in next to you, you're going to have to face the situation sooner or later. If this is something that she has done repeatedly then bring her up to standards or morals or chapter relations or whatever you call it, but don't rally the house around your cause, that's just not cool.