Most Helpful Guy
You can undo what has been done. It sounds as if he does not want to reunite and, even if he did, doing so is almost always a mistake.
You should talk to your sister. She probably thought that she was helping you but I'm sure you've heard about the road to Hell. Explain to her how her actions potentially affected your ex and you, so that she will understand why her interference was wrong.
Remember, boyfriends will come and go but she will always be your sister, so explain this in a nice way and don't put her on the defensive.