Well, everyone dies eventually... lol... but Sandy is going to go into remission (there's a slim chance I'll change my mind on that, but... it's very slim). Ryan and Sandy will definitely manage to mend fences, but I don't know if Ryan will ever be able to forgive Kirsten and Seth, especially Kirsten. I know that seems odd, to forgive the person who actually hit him and not forgive the two who didn't, but... the key thing is, Sandy shows remorse for his actions, he's well aware that he caused things to go to hell, that he hurt Ryan, both physically and emotionally. He knows that he needs to earn forgiveness, that Ryan owes him nothing, and that he will be damn lucky if Ryan does forgive him.

But Kirsten and Seth are both in the mindset that Ryan 'abandoned' them to deal with the situation. Kirsten won't acknowledge that her affair added to the whole mess of a situation, and neither of them seem to understand that Ryan was a kid and he was trying to hold things together, he stayed two weeks longer than he should have (he should have left after the first incident, instead of waiting until it was happening again and someone else convinced him to leave), and that he is in no way at fault for what happened either before or after he left. Seth is consistently selfish and forgiven for it, but the one time Ryan thought of himself first, and Kirsten and Seth are villianizing him for it.

So, in short, Ryan will have a relationship with Sandy again. It won't be like it was, it couldn't be after everything that happened, but it will exist and it will be a good relationship. But it's a lot less likely that he'll have any kind of positive relationship with Kirsten and Seth.