I really don’t think you understand how much I love “shitty older brother Cronus.”
HOLY SHIT this is the best thing ever.
I bet they have the shittiest parents. Total flakes that are gone all the time. They're so disappointed that Cronus stopped making the honor roll but don't put in the effort to find out what changed or what they can do to help. And poor Eridan. He only has Cronus, but only when Cronus is around. He feels bad about leaving Eridan alone as often as he does but not bad enough to stick around more than he does. It's just hard to when your brother asks why mom and dad are gone all the time because you realize you don't know. Maybe it's you.
So Cronus hangs around his brother more than most of his friends hang around their own siblings, but wow is he a shit brother. No patience for questions about life or school or friends or romance or anything. He makes sure Eridan is fed and sometimes drives him to things he has to go to. He's a dick and curses a lot. He's not very good at making Eridan feel better after a shitty day and, honestly, he doesn't try very hard.
But he is around.
When Eridan is in his mid twenties and seeing a therapist about all his troubles he will come to the realization that all his problems are not his brothers fault. His brother wasn't great but he was around. His brother was there. His parents were not.
It will dawn on Eridan that he was expecting Cronus to act like a grown up when he was going through the same things. He needed someone to be there for him too.
When he gets home he'll call his brother and make the first meaningfully sincere apology of his life.
"Cronus, I'm sorry I've been so bitter. I always expected you to be responsible for me, and that wasn't fair. You weren't my dad. You were my brother, and you're a really good brother, Cro."
Cronus, in his moderately successful artistic career with his string of failed romances of all kinds and no friends to speak of will just say "thank you" and that will be it. He will go to bed early and cry out of relief that he has not dicked up the only relationship he has that seemed to stick. Not all brothers are forever, but his is.