OP
I do not think you need to forgive or understand your mother in regards to her parenting of you/the way you were treated as a child, which sounds like it was neglectful and inadequate. Your resentment sounds like it is justified, but hope that have help to come to terms with it, as much as is possible.
Your situation with regards to your first child is tragic and I’m so sorry that happened to you. Again, I hope you have had/continue to have whatever support you need to come to terms (if that is the right phrase) with it.
I understand that you don’t blame your mother re the adoption of your brother, rather you resent that she did not tell you about him. If you’re angry, your feelings are valid- you feel how you feel. However, your feelings and wishes do not invalidate your mothers wishes either. I think, in this one regard, you perhaps judge unfairly. By all means resent your mother for being a poor parent, and maintain whatever degree or manner (if any) of relationship with her that brings you most peace and stability. But I do think you judge too harshly her decision not to tell you about your half brother).
I think it is entirely understandable that your mother would want to try and blot out /not acknowledge something that she probably finds very painful, quite possibly feels at least some degree of guilt about, and may well even be profoundly ashamed of, for a whole variety of reasons. Telling you about your brother- however understanding you might have been (was she to know that you would be?)- would mean thinking about it, talking about it, admitting what had happened and giving you information about what was most likely a difficult time in her life. It would be intensely difficult for most people to open up those wounds again. I can understand entirely why she would want to avoid doing so. I understand you have always been open about your son to your children. I know this is what is usually advised now. But it wasn’t necessarily the case when your brother was born. Your mum’s natural instinct to bury it was matched, by the sounds of it, by significant encouragement from her father who exerted a lot of influence over her decisions (according to you). Given her sisters advice, I imagine the wider family may also have preferred to “leave it in the past”. She had, therefore, lots of reasons not to be open with you. I’m not convinced your wish to know trumps her rights.
If I were to discover this about my mother, I genuinely do not think I’d feel aggrieved that she hadn’t told me. I think I’d feel profoundly sad that she had been in that position.
We have had similar in my own family. My aunt had a baby at 16, the decision was made that the baby would be adopted (not a decision that was entirely her own, she was strongly “encouraged”) and she was sent to relatives away from home to have the baby so that few people would know. Few people even within the family knew. I understand that it was many years into her marriage to my uncle before she told him. They did not tell their children. It was only when her first child tried to make contact (and stated they wound contact relatives via social media) that it came out. It was a very difficult time for my aunt, for whom the stress of the whole situation triggered significant mental health problems from which she has yet to recover. Very sad situation. I don’t for a single second sit in judgement of her decision. Nor do her children.
I suspect that, has she been a better mother and if you had a better relationship, that you might feel a little more understanding.