When you know someone cheated..

So my bf and I recently moved out from living with my sister. About a week before we moved my sister and I went to a concert and our bfs hung out. We didn't leave on the best of terms and my bf made a comment saying she's getting what she deserves (He wouldn't tell me what he meant by it).. My bf told me the day we moved out that when him and my sisters bf hung out (while we were at the concert) they were in a spa with friends playing truth or dare and my sisters bf made out with another girl (which is one of her closest best friends). My sister is a bitch sometimes, it's literally a love hate relationship. Should I tell her or not? If I do her bf will know my bf told me even though he said he wouldn't say anything.