So your sister made a super dickhead move and stole your cheating, asshole boyfriend. In the long run, she probably (unintentionally) did you a favor. In the meantime, she'll get to find out that what comes around goes around (no, I'm not in *any* way saying I want her to get abused, but it seems almost inevitable that she'll have an unhappy relationship and eventually get cheated on herself). It sounds like they deserve each other, in the worst way.

&amp;#x200B;

Maybe someday in the future you'll find it in you to forgive her, but the *if* and *when* will be completely up to you. She fucked up a lifelong relationship with you for a fling.