Well, thank you for reaching out, Dear Soggy - I really hope it helps to get it all out there.
You have every right to be disgusted and dismayed (aka "bitter") over your toxic family members. Indeed, there would be something absolutely wrong with you if their treatment of you didn't affect you to this day. So, please cut yourself some slack and don't assume it should not be affecting you at this stage of your life.
Your mother is not only a malignant narcissist but when you initially confronted her about your brother she was forced to confront just how deep her mistake revolving around his existence was. No fault of your own, but I'm sure she felt like she was "getting her nose rubbed into it", so to speak. That is especially unforgivable to a narcissist. Be ever so thankful you've gone no contact - the very best thing you could do for yourself even though it's painful.
Forgive yourself for all the acting out and focus on becoming the best you you can be. No perfection is ever expected and personal healing and growth are a journey. No magic pills and it's generally not easy. Sometimes there are leaps and epiphanies, more often baby steps,'and, yes, a step back here and there. But, the path is always forward when you're willing and able.
Society, movies, self help books - they all preach forgiveness. Don't fall for it! What I mean is don't beat yourself over feeling the way you do towards your mother and brother. You are justified. As you heal, and if you are so inclined, accept. Forgive - or no! Let the focus be on you and your daughter, the good that IS and the good that is to be.
You are in therapy, I hope?
There are some wonderful people on YouTube who produce channels on narcissism. It's one thing to finally put a label on these N-people, but it is so helpful to learn more in order to process the trauma you've been through.
I hope others here can offer their perspectives.
Hugs!