Welcome, DillytheGardener! My heart goes out to you and all the heartache you've suffered - and at the hands of your own mum and sf (stepfather). I don't think you're in the wrong for those periods of estrangement. Imo, you're protecting yourself from further hurt. And maybe, it's also your way of expressing your anger at how she treated you as a child. You say she tells you she "doesn't like" you. Have you ever told her how much she hurt you all these years? If so, what was her response?
Actually, I'm a little surprised you let yourself be exposed to more hurt by jumping in when there's a crisis. It's very brave and unselfish of you, but why is it that your brother, the "Golden Child," as some people call the favored one, gets away with not helping at all? What would happen if next time, you said, "Sorry, I can't be there this time/can't do anymore/etc?" He lives there, maybe let him take a turn at helping her. Or do you fear he would just let her suffer?
Rock and hard place, I guess. Keep mum out of your life, no matter what, b/c she has hurt you so much vs. be there for her in a crisis b/c, hey, she's still your mum. I get the sense of obligation to help her out (too bad brother doesn't seem to share it). In the future, though, if you're going to spend a lot of money, anyhow, why not hire someone to come in and take care of her? And why doesn't she pay for it instead of you? Or at least, split the costs? You shouldn't be spending your savings, surely. Hugs!