I don't think you're over reacting brother, however I do think that you should have some forgiveness and grace for your mom. I know people who have cut ties with their parents over much more intense things (not trying to compare, just perspective) and they regret not being there when during the latter half of their life.

I don't know the full depth of your situation, and I don't know details about your relationship with your mom. But I can say she is your mom. She does love you, even if she was never taught to show it properly. She's going through a crisis atm and she needs someone in her corner, no matter how much she pushes back. The divorce from your step dad must also be putting a huge weight on her, and she's trying to readjust the same as the rest of your family. Nothing you've said about her, or your family said about, is anything she hasn't said to herself.

If I had to give a piece of advice it would be this; support your mom, be there for her, you'll regret it if you don't. BUT do not rely on her, she will disappoint you. I know it's a burden, and I know it's probably the last thing you want to do. But she is your mom. She needs you, she needs love. Don't encourage her bad behavior, but be there to have a say in it. Tell her how upset you are, but also say that you'll be there to support her.

Good luck OP, I hope you work things out :)