Yea, helping Suzuka is the right thing to do and honestly? I'd go beyond petty, if I was in Ryouta's shoes I would actively tell her why I can't be seeing her much after this, morally it's somewhat wrong to hit Suzuka with knowledge like that but frankly those two don't deserve her love and affection, Suzuka also deserves the truth.
Frankly I really hope in some way these two "parents" get punished somehow, because I have a gut feeling they won't get anything close to punished.
I said similar before, but I do acknowledge that Suzuka's like 6 years old and in her short life has only known love and affection and support for her parents. I actually need to go re-read, because I'm not certain she even knows that Ryouta is her
blood relative, and just thinks of him as the kind older boy who helped her out and is visiting her and being nice. She's not privy to all the drama and history that Ryouta is, and I'd argue that she's far too young to embroil in what amounts to a problem between him and their parents, specifically.
That "blameless victim" quality of Suzuka's encompasses all of her circumstances, in that sense. And to that end, I actually think a potential "ongoing punishment" for the parents, were the author to go that direction, would be for Ryouta to actually be a viable donor for his sister, for him to go through the procedure and have Suzuka ultimately be saved, and then simply not see her again, cutting off all contact.
So that every time Suzuka says "Where's Ryouta-onii-chan, I wanna see him", her parents have a choice: they can lie to her (and carry that lie that compounds their shame in having to hide the truth), tell her some version of the truth and have to deal with her internalizing that as she ages, or hide it entirely, which is akin to lying and largely parallels the first option.
But either way, we effectively have confirmation that the Ryouta and his dad were correct in that his bio father never wanted him, and that his mother effectively chose her AP over her child. Even with her "but Ryouta's m—" partial line from chapter 2, the fact remains she could have fought for custody in court, or
something, and she ultimately chose to abandon Ryouta after conceiving him out of infidelity with a man who didn't want him at all. To go on and save the life of the child they
did want, and then simply walk away and leave them to deal with answering questions, is the sort of backhanded good-samaritan behavior I can applaud.
Besides...Ryouta can write all of them out of his life after doing the objectively-correct thing in saving Suzuka's life, and go on to have a happy Found Family and marry his stepsister. Because as some would say: "living well is the best revenge".