How do i break up the friendship between my ex girlfriend and my sister?

My ex girlfriend cheated on me with another guy at work I find out after reading her text I decided to catch her in the act and I did and we broke up I was devasted and depressed but I got over it my sister knew my ex before me she was the one who introduced me to her she hooked us up I always went to my sister for advice she's my older sister and we always been close. Anyway my sister invited me over her house for dinner the other night it was just me, my sister, brother in law and Surprise surprise my ex girlfriend. It was really awkward she knew we broke up my sister told me that they are friends despite what happened to me and her now I feel bretrayed by my sister I told her to never speak to me again its either me or her I am really upset that my own flesh in blood would do this to me how can I break up this friendship between my sister and ex I don't want her around my family she's a cheater and a whore please help I'm really desprate.