What an awful situation.
Ok, here are my 2 cents. Do NOT talk badly about her to your brother. Or your mother. In fact, don't talk about her at all. Your brother is a grown man (even if he doesn't behave like one). He got himself into this situation and now he has to deal with it. i know it's hard, when you feel like he is being taken advantage of, but there isn't anything you can do but bite your tongue, let him make his mistakes and then be there for him if it does all go pear shaped.
Be polite to her when you do see her, but try to keep your distance. Maybe even delete her from your facebook so you don't have to read those things about your brother. In other words: stay out of their relationship. If you don't, you could further alienate your brother and strengthen her grip of him.

Talk to your brother about your relationship with HIM. Not with HER. Leave her completely out of it. Tell him that you were hurt that he didn't come to your son's birthday. He is a grown man and makes his own decisions. So even if his wife doesn't want to come, he still has to own his own decision of not coming.

I don't know if this comes across the right way. It's hard to put in words, especially when you can;t hear the tone in my voice.

My issues with my SIL took a turn for the worse with her "unplanned" pregnancy. I am 100% sure that it wasn't quite so unplanned (things she let slip, circumstances). I felt really betrayed. My brother adores her. My family had accepted her as one of us. And then she goes and does something like that. It took me a while to get over that. But in the end, i knew that she actually loved him. She had lots of issues from her childhood that explain what she did. But they don't excuse it. And most importantly, I know that he loves her. And he's happy. And that's all that matters to me. He ended up proposing to her, marrying her and they have a gorgeous son. He is deliriously happy and I know he would have married her eventually anyway. The pregnancy just sped things up.
She actually felt my negative feelings against her and it didn't help anyone.

Anyway, I know your situation is different as your SIL doesn't seem to love your brother. Just thought I'd let you know that I sort of understand your situation.