Lil Sis crossed the line

[OPAGal]

I recently went on a leave of absence from a national service sorority that I belong to. I went on leave for a number of reasons, but one of the main ones was that my sisters began having a negative attitude towards my lil sis(bad comments about her weight in front of her, ect.). She was also suppose to taking a leave so that things could cool off then about a month ago I found out that she never took a leave. I kept wondering why that she never called or anything this semester. She had started running around with my BF's Big Bro's girlfriend. (Hope you can follow that) And she told her that she just couldn't wait until my BF of five years broke up with me so she could have him. I am in complete disbelief. I can't believe that she would actually do that to me! I've been so nice to her. Right now I can't wait to get ahold of that girl! Can anyone tell me a rational way to handle this before I just choke this girl!