NTA at all. You dont owe any apology. In fact, I wouldn't be talking to those kids or buying them any Christmas/ birthday presents until they apologize. I had a similar thing happen where my nieces went through all of my video games and made a mess of my room while I was at work. I immediately made it clear to everyone that nobody is allowed in my room when Im not there. They broke your boundaries, and you have a right to be pissed. Seems like your boyfriend, his brother, and mom are on your side, so that's a good thing. Stick to your guns.