I think the best outcome is Ryouta agreeing to at least test if he's a compatible donor for his (actual) sister, and then if he does in fact match, then agree to the relevant procedure that would save her life. Hopefully whatever treatment involved is a one-and-done and doesn't require repeat procedures or a regular schedule of injections or something, too.
And that's it. He doesn't have a relationship with her whatsoever, and their parents have to explain to her exactly why her big brother isn't in her life, and how it's all their fault. Bonus points if she rejects them too, but they'd have to live with the shame, either way.
Ryouka owes none of them anything, but Suzuka is innocent and none of the things their parents did to Ryouka are her fault. And I expect Ryouka to realize that her suffering wouldn't fix what was done to him.
And honestly, saving her be a moral victory of his over his mom (and bio dad). It'd be an extremely petty reason, but being able to lord that over the dad (and mom) would be a pretty compelling reason for me, vindictive as I am.
I hope this doesn't go down the road of absolving his actual parents of their nonsense. They abandoned him, so while I do think he should save his sister, I hope the author doesn't force Ryouka to forgive their monstrous actions, and they're made to live with that.
I do wonder if Akira's dad is gonna die, though.