My brother has two kids and a wonderful girlfriend. As much as I love her and their two kids, and want them all to be a family, part of me thinks she should just run. He is my brother and I love him, but he is just so mean. He wasn't always so unpleasant,in the past few years, it's gotten increasingly worse. It's always about how hard he works, and his struggles and he accuses his girlfriend of basically being useless. I've noticed that he is very jealous. He doesn't let her have any friends really (except for his, when he is around), and criticizes her about everything. He even criticizes her in front of their two young (2 and 3) children. It's wrong. They just found out they are expecting again as well, which he of course blames her for. It's not just her he acts that way with. He is very mean-spirited to me and my mom sometimes as well. I think he should talk to someone about why he's so hateful, but that's something he would never agree to. I just feel awful because, while I love my brother and I don't want his relationship to end, I feel terrible that his girlfriend is a prisoner in her own life. I just don't know what to do.

What you do, all you can do, is tell her privately that you are there for her if she needs support.
And then you back off.
You could even talk to him privately to offer support. He may indeed work very hard, may regret having kids, may be mean because he feels that he is honor bound to something he doesn't want.

A couple should be talking equally about the number of children they have. But we don't know the whole story. If he blames her for the 3rd pregnancy, she may be saying she is on the pill and isn't, or he may think men have no responsibility, or he may just not care about facing the future and likes to blame after the fact. It may not be his child. It's no one's business but theirs. But we can't judge on the surface based on what we think we see and know.

I know for a fact it's his child. She is on the pill, but she missed a few days, but she told him that before anything happened. He knew. And I have tried to talk to him in private, but he is the type of person (unfortunately) who thinks he is always right and doesn't listen to anyone. He's impossible.

As his sister, I bet you have tried talking to him asking why he is mistreating his girlfriend.
Somehow he needs to wake up, is there anyone that he respects so much, within the family?
Also, the girl should not give herself in for whatever she is being accused of. There is a proverb in my native language which goes like "the more you keep bending only to get whacked more". She has to ask him to stop being mean. Or who knows, may be she knows what the problem is. Hear out her thoughts and give her some support.

And... you've talked with her? Without meddling, just to be there if she needs you?

I'm not trying to sugar coat him, but I'm finding it hard to believe that you know the exact events of she 'missed a few days' and 'she told him.' Were you there? No, of course not. So whose word on that are you going by? If not both of theirs, then you really don't know the whole story of her irresponsible birth control. Sometimes women keep getting pregnant as a way to hang on. I wonder just how many days she missed, and why. You just don't really really KNOW what you think you know unless you are there, and even then there is often back story that is missing.

You say he is impossible to talk to because he is always right, but you didn't say you talked to him to offer support. Support means NOT telling someone what to do or where they are right or wrong. It means listening. Since their marriage is none of your business unless he's hitting her, of course he's going to be impossible to talk to. I would be too.

So again, she needs an offer of support (listening) and he might be willing to accept just support too. Support doesn't mean you agree with him, but it does mean that you keep it to yourself.

He obviously has some problems but you won't change him. Just be supportive to her and the kids. If there seems to be a pattern of abuse(it sounds like there is)she needs to deal with that before it affects the children. Some people are just mean and angry about things. You don't mention his age or what he does for a living. He could be extremely unhappy or very highly stressed.