nail on the head
You pretty much described my ex when you described your brother. Both his wives had/have needed fixing/rescuing. It was hard for him initially that I didn't, that I was strong and independent, which he'd never experienced; but, he also feel in love with it, because for the first time, he wasn't someone's whole life.
It is also, however, why he chose to give in to her. I don't need rescuing and am very definitely the strong one, so the need and demands of the "weaker" one come first in his world. It had actually been a problem for some time, and I kept fighting it (it is also the primaryreason she and I didn't work: I don't handle that kind of emotional fragility in others very well, which I realize is my problem and not anyone else's). This last incident, which precipitated the break up, was definitely caused by that.I also don't believe it is ever someone else fault or responsibility when an adult chooses to self-harm (or threats of self harm) as a way of acting out. We cannot control the actions of others. He, however, believes it's his duty to not let it happen, and for his sake I hope she doesn't do anything (I don't think she will, btw). To me, those kind of threats are emotional blackmail, and I don't negotiate with blackmailers. But, I am not in his shoes, I am not him, and he had to do what he believes is right. It's one of the reasons I love him.
IT is his choice, but I hope, some day, he can find what he deserves in life, whether it is with her because she's changed, or some other way. If they broke up in the future, I would date him again. I know he would like to see me again. But, while they are together, I couldn't. We'd just end up here again I think, and there is a point at which I have to emotionally protect myself. I've never felt the live I do for him, and that joy ifs reflected pretty accurately in the amount of pain I am in now. I can't pour muddled here again knowingly.
He deserves to be happy, and I want that for him. I am trying to be adult enough to wish that for her, too. At the very least, I wish her strength and emotional health enough to help him be happy.