I thinking of cutting off one of my closest friends in college.
Is there a particular reason why you would do this?
Yes there is. He was having sex with my sister all of the time, he took advantage of her.
It sounds like this is something between him and your sister. You should not cut them off if the rest of the relationship with them is fine.
I admit that I'm going to cut him off just because he was having sex with my sister, but I still want everyone in the school to find out the truth.
Who your sister and friend dates is up to them. You can try to tell them how uncomfortable it makes you feel. If you break it off with them, you'll still see them dating your sister, so nothing will be accomplished that way.