My brother is going to get the snot beaten outta him by my BF. What to do?

Wow. Well first of all your brother sounds like he hasn't really been on his own. No offense. As to your question if i were you I would ask your BF him to please not hurt your brother to bad. Your brother sounds like he needs a good dose of reality and perhaps realizing that just because he is a big boy doesn't necessarily mean he is invincible and he can be taken down. Your brother might be a better man for it. I hope this helps. Good luck to your boyfriend by the way.