Don't you dare let them try to twist their lack of a spine into your fault, Squirt!
Stuff like this always comes out. Always. It amazing you kept silent for so long. So much to keep to yourself for so long, poor dear!
I completely agree with Vash. You mom is dealing with her guilt over being a shitty mom. She knows you know, both for yourself & now as a parent. That's going to haunt her the rest of her life. Pity her, if you can. Forgive if you've healed enough within yourself. But don't expect your family to understand or to have changed. Obviously, if they had that capacity, then the abuse probably wouldn't have happened in the first place.
You, on the other hand, are a great mom. You've done all you can for Kyle, including tough love when he needed it. Of course she will resent your success. It takes a really big person to be happy for someone elses success in the face of their own failure. Again, a person like that probably wouldn't have let you be abused in the first place.
We have a couple nutbars in our family (what family doesn't?). The best way to handle them is to think of them like rabid animals. Interact with them if you must, but protect yourself & set your expectations accordingly. Stop trying to be a peacemaker for a family that doesn't appreciate it.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh