Black Brothers
It's Sirius' fourth year, and he and Regulus are drifting apart. The only time they ever communicate is during the holidays and even then, Sirius spends as much time as possible at the Potters. He's tried reaching out to Regulus at school, but he's always with Barty and Evan, and they are downright aggressive when it comes to Sirius. He's still desperately trying to mend their relationship but he has a feeling that Regulus has given up and now he's just grasping at straws.
It's a couple weeks after the Christmas holidays, which Sirius spent half of at the Potters. He had invited Regulus, but he didn't even bother to respond. Sirius is in bed sleeping when he is awoken by something hard hitting him in the face. When he finally has his wits about him, he recognises Barty and Evan standing over his bed, glaring at him.
He asks them what the fuck they want and they say Regulus had a nightmare. Sirius is wondering if this is dream, trying not to laugh at the absurdity, but they continue. Evan says that Regulus would never ask for Sirius to come stay with him, so they did it instead. Barty threatens to skin him alive if he doesn't comfort Regulus to the best of his ability right this second. This is the point when Sirius realises that Barty and Evan don't hate him because he's a blood traitor, they're just very protective of their friend.
Sirius obviously agrees, and heads downstairs barefoot, still in his pyjamas, registering that the Slytherins are too. They must have rushed to his dorm. They make it back to the Slytherins dorm, and when Regulus finds out where they went, he's furious, but he still doesn't object when Sirius offers to sleep with him for the rest of the night, like he used to when they were kids.