Matt/Jenny/Abby
They're at the top of my list, because it's not a coincidence that she was shot right after they threatened her, but you're right - there's a possibility someone else could have taken advantage of the very public argument to take care of things themselves, thinking we'd look at them first. I really don't see the sister being behind it right now. She's too convenient a target for having done it, and I don't like when things fit neatly into place.