Just having observed many of my friends who have divorced, and then one or both exes re-married, where there were kids involved, I have seen what seems to work, and what really doesn't. A few of my friends have remained friendly/respectful with their exes and the exes' new spouses. I'm sure it really wasn't easy for them to be friendly like that, with the hurt feelings that were probably involved, but I do have several friends who did it, and their kids are SO much better off and well-adjusted for it.
As an example, we know a couple where the wife had 3 kids with her ex, and then divorced when the kids were pretty young. She re-married and had 2 kids with the new husband. Her ex-husband re-married and had a few more kids himself. The kids mostly live with their mother, but do spend time with their dad and step-mom on holidays and during the summer. The kids refer to the step-mother as their "bonus mom." I thought that was a nice way to refer to her - and it was at their mother's insistence that they call her that. She wanted them to feel that they didn't have to choose - that they could love her, their mother, as well as their dad's new wife, and that when they were at dad's house, they should feel like the woman tucking them in at night was someone who loved them just like they were her own. Likewise, the step-dad is referred to as "bonus dad," and he sees no difference between his wife's 3 kids, and his own 2 kids. They're all his kids in his eyes.
Now, like I said, that can't have been easy to come to that point for the adults. But these kids are SO well-adjusted. They're teens and going off to college, and every one of them has straight As, lots of friends, and are active in sports, their church, etc. These kids are a credit to their parents (and that's what we all want for our kids, right?). Every adult in the situation put their own hurts and issues aside and just embraced the fact that the new wife (husband, etc) was there to stay, and would be a part of the kids' lives - so better to make the best of it for the kids' sake.
If you are able to do this, I think your child will be better off. It's really not about you feeling hurt that this woman doesn't acknowledge that you were in her husband's life first, or whatever, it's really about your child. Your child now has a brother who will be his brother all his life. That's not going to change. Put your hurt feelings aside and try to make nice with this woman, who is your son's brother's mother, and always will be. You don't have to be her best friend, but stop fighting for your son's sake! Don't make him feel like he has to dislike this woman. What is the point of that? You're his mother and always will be. And now he has a bonus-mom who can be there to back you up. You may not agree with every single thing she says or does, but who cares. She took enough of an interest in your son that she got his father involved in his life, where he hadn't been before. Right? She sees the importance of family and is making an effort. That has to be worth something, even if you don't want to be her friend.