LilQueenie - I feel for you, I really do.
I am in a similar but nothing like as bad situation as you - DH's brother lives with MIL 15 mins up the road and has a drink problem, plus social problems even when not drunk - but no actual mental disease, so he can't be medicated. And even if he was, he wouldn't take them because he thinks he's fine, it's everyone else. His sense of entitlement is colossal as well.
We actuall have a restraining order on him, keeping him away from our property and if we go to MIL's and he has been drinking, technically he has to leave as he is not allowed near our DS (or DH for that matter) with alcohol on board.
MIL is a PITA about it all but I think she feels guilty for "creating the monster" by not being strong enough with him - she lets him get away with all sorts and just continues having him live with her, even though it means we rarely go there in case he's there.
Sounds like your DP's parents feel guilty about the injury; and that your DP can't leave because he feels the need to be there for his parents (understandable).
I thikn though you have to do what is right for you and your baby. I was "lucky" in that the police themselves pushed through the restraining order against BIL - DH was going to withdraw it if possible because it "upset his mum" but the police told DH he'd be done himself, possibly for contempt, can't remember if he even tried it, becasue DS was involved.
So far, DH has managed to do the right thing re. calling the police etc. - I would find it very difficult to do it if he didn't want me to though so I can see your dilemma there - but you have to log your concerns with someone and try to get a restraining order against him so that he cannot come near you nor your baby, whenever he/she is born.
Good luck - it's a horrible position to be in.:(