I've just been musing on this a bit further OP.
I think it's quite easy to frame this as an all-or-nothing situation. As I said earlier, I think you'd find therapy really helpful but all of that aside, perhaps you need to have stronger/clearer boundaries with your mum.
My mum is a really difficult character. My situation isn't remotely like yours but I couldn't give details without a name change, lol. About 17 years ago I very nearly went NC. For many, many years, I was LC. Very LC. For context, my brother is NC with her and has been for 10+ years.
Bizarrely, my mum now lives in my annexe. If you'd asked me 10 years ago about this scenario, I'd have laughed in your face.
I think getting older helped. I managed to let go of some of the resentment I was carrying because I think it just seemed to matter less. I had my own DC and they became my focus.
DM was still bloody difficult - and she still is. What's changed is that I recognise I don't need to be drawn into it. She has mild/moderate dementia now so she's even more difficult on some days.
I have very clear boundaries that I'm not shy about setting out. There are some subjects that we're never going to agree on, so we don't discuss those. I don't want to hear about them, I don't want to hear her complaining. We have a locked door between us and I provide support to her on my terms.
I'm glad I didn't go NC because I think it always would have gnawed away at the back of my mind. I'm not the kind of person who could have just let it go peacefully.
It sounds as if, other than the issues with your brother, you have a good relationship with your mum. Maybe after some therapy, or even before depending on how you feel, you'll be able to sit her down and tell her calmly that the situation with your brother is seriously affecting you. And the only way to continue to have a relationship with her is if she doesn't talk to you about him, nor complain about the fall-out. Those are her decisions as a parent, you don't agree with them, so you won't be complicit by taking on the emotional labour of hearing about them. And even the slightest hint of those terms being broken, you shut the conversation down instantly, and move on.
You just need to get to that point emotionally where you feel OK with that as a resolution. And it depends whether that's right for you, it won't be for everyone.
It's just an alternative to NC/LC which might be something to aim for 💐