Drop them. Seriously.
I cut out my father a few years ago, and my cousin was recently jettisoned, too. One of my kids loves my mother very much. If it weren't for that, I'd gladly cut contact with her, as she's very like my cousin - loves to create drama, endlessly martyred as a cloak for narcissism, the same pain in the arse mould your mother seems to be made from, in fact. And yes, I was also abused by a family member as a young adolescent, and my distress and struggles with that were seen by the family as a whole as my being the problem. My family hate it with a passion if I ever refuse to pretend, along with all of them, that it never happened. I pretty much despise them. Have you had counselling on this one? It can be very helpful in reframing the victimisation sadly often dealt out to abuse victims in families, so you realise the issue is with them, and not you.
Don't worry what they think of you. Their behaviour has plainly demonstrated that their opinions are worth less than yours. Your mother enabled a paedophiliac rapist rather than give him up, and your brother enables her because he likes being her golden boy, and so scapegoating you suits him beautifully. Your ex sounds like they are at his level. Leave them all to it. Don't react or respond to your mother, because she is not capable of a reasonable response. Just walk away. Block her, don't meet with her, don't see her, don't engage. Drop the rope.
You cannot reason with an unreasonable person. You cannot make a woman who feels marriage demands sacrifices, and sees no problem with that sacrifice being her young daughter, normal. Or create a conscience in people who tell themselves whatever they want to hear, and who enjoy manufacturing drama because it validates them. All you can do is walk away, and be grateful that you have better and more positive uses for your time and energy.
I haven't engaged with my family when quietly removing them from my life, because if they were capable of a constructive engagement, then I wouldn't need to remove. You don't owe anyone your time, let alone your affection. Genetics doesn't entitle people to use you as their scapegoat, punchbag, or resource. Relationships that aren't genuinely reciprocal are not the other party's prerogative. And your mother is plainly not just a taker, but an abuser, too.
I think you should bin the ex and the mother simultaneously, myself. Both harm you. Neither offers anything but pain. And in my own case, I felt nothing but lighter, with every toxic relative I have removed from my life. Why let them sap your energy and happiness? Enjoy your dds, and your friends, and your work. Your life. Free.