Annie, I'm so sorry this happened to you. As a quilter, I can appreciate the time, thought and effort that goes into making a quilt - even when it's just being put together. To have someone destroy it for no reason is unbelievable.
There are lots of great comments here. My two cents is to not have them over again and I'd rethink my relationship with my BF. Have a serious talk with him because when you marry someone, it becomes a package deal.
I'm shocked she just lets them do anything they want and it sounds like their father doesn't even step in. What's up with today's society?
Good luck to you!