Sirius would do anything to protect his little brother Reggie. He sacrificed himself, again and again, for Regulas, always placing himself directly in harm's way so that Regulas could escape pain. When the time came for Sirius to go to Hogwarts, he hadn't wanted to, not if he couldn't bring his brother with him. Sirius' young self had known subconsciously that bad things would happen to Reggie if he wasn't present. This inkling of a feeling was proved more substantial when Sirius received the letter detailing his little brother's ascension in Lord Voldemort's ranks. Regulas had become a death eater.