...This bro.
Seriously you started it by not just going out with her best friend(honestly, that should have been looked over - you should have said 'hey the slashing of my tires.. what was that about? O.o'), but you destroyed your sister's personal love life. That goes way beyond vindictive ex's. Let's not forget to what extent you went to do this. Tires are several hundred dollars. The therapy/emotional trauma you probably caused is worth way more then that.
Basically, call your sister in and tell her it's time to have an emergency meeting. Figure out where you two stand. Get closure if you *really* want to shut everything down. Be a legitimate couple if you don't. Stop being babies and acting out brashly. It sounds like you two need more help then just closure if you attempted to fuck her friend to get back at her - and she decided to slash tires before having a pointed,calm, sit down and face to face heart to heart talk.
