Christ mate that sounds awful, I hope you and your family sort this out. I don't know what to suggest, when you're with someone they can get into your head like no one else being 'in love' has a powerful hold over your brain (trust me). I can imagine having a baby makes that even worse - I wouldn't blame your brother. If she's in his head and telling him that if he comes back to you he won't see his kid, or she'll kill herself or anything like this his natural instinct will be to protect himself and the baby.
If I were you, I think I'd keep texting him messages of love and support, even though he might not reply you know he reads them as he replies to the angry ones. When he feels he can do something about it, he will come to you because you've been there for him, even just via text. If he does text back try as hard as you can to meet with him, say no parents or girlfriend and no arguing just a hello. If you meet him and just be nice to him he might let it all out, he's probably desperate for some help but can't actually take the step to getting it. I hope it works out for you all.