How do you think Sirius and Regulus reunited at Hogwarts after Sirius ran away?
Asking me this on my starcest sideblog means you get a starcest answer btw...
I'd like to think that Sirius ends up pushing Regulus against a wall in one of the castle's many winding hallways, arm on his brother's throat because he needs to speak and Regulus needs to shut up, telling him that he'll never forgive him if he ends up dying. He rips himself away from his brother before doing something stupid like kiss him. But he makes his brother understand that his words are a threat not a reassurance (there's no affection to be found in his words). He can't be choosing himself by staying only to join a war and die — Sirius simply won't allow it. Nothing would make him hate his brother more than if he were to actually die. He lets Regulus know.
Regulus almost nearly claws Sirius' eye out at his brother's audacity because Sirius has no right, to anything, anymore. Much less what he does to his life.
This, in line with a canon compliant take from me, would just be irrefutable distance from both sides. Like, we have to remember that these two die away from each other and are not on speaking terms :( Sirius doesn't even know what the fuck happened to Regulus.
Going with my headcanon that Sirius asks Regulus to come with him before he leaves at a point in their relationship where they are fundamentally at odds with each other, their meeting at Hogwarts following this would just be... Them not being together.
Regulus is still reeling from actually being left behind (despite refusing to come with Sirius. The point for him was that he never wanted Sirius to leave, not for Sirius to ask him to come with him). And Sirius is just as hurt with Regulus choosing to stay, thinking that it's as good as abandonment — him leaving alone.
When you look at it this way, both Sirius and Regulus are choosing their own safety and it looks different.
Do they fight once they get there? Does Sirius follow Regulus to antagonize him? Does Regulus refute Sirius' attempts at reaching out? I don't know. I'm not sure.
But I think that their relationship dies into a painful, ever yearning heartbreak the moment they stop living in the same house. Even moreso once Regulus gets the mark during his sixth year. There's definitely enough time left for them at Hogwarts for one off encounters, I imagine they'd be tense or maybe fraught. No, they are not okay. Yes, it hurts like hell.
Even at odds for most of their life they've been so used to having each other, having the other at reach. Sirius leaving is a part of their relationship where the distance is real and it means something. When they meet, they meet with all of that love and hurt swirling between them.