Am I overreacting?
Most Helpful Guy
You were with him for 6 years?
That is crazy that your sister would do that. It would be shitty of any of your friends to do something like that, let alone a relative.
You can date exes of siblings if it was a super casual relationship that ended awhile ago, but come on!
It sucks, because I have no idea what you can do. I suppose all you can do is go to your sister about it and just present it plainly:
You aren't ok with this, you likely never will be. Part of the healing process if you breaking up involves cutting him out of your life and she is preventing that from happening. She is putting herself in front of you and that makes you feel like you don't matter, on top of being hurt by the action itself.
You can't really take any action to stop them, but you do need to let her know what is what.
Most Helpful Girl
I don't think you're overreacting. There are rules for this. Bestfriends aren't allowed to date your ex, mother or sister. Same goes for a sister, she's never allowed to date an ex, especially w/o talking to you first. I'd feel betrayed and hurt. If I were in your shoes I'd probably stop talking to her. But that's just me. I'm sorry this happened to you :/