Sirius Black never got along with his little brother Regulus. Perhaps it was the fact that Sirius was only two years old when Regulus was born, and he didn't understand that his parents had to spend lots of time with the new baby, not him. Perhaps it was the fact that their mother, Walpurga, showered Regulus (who looked a great deal more like her father than Sirius did) with more attention. Perhaps he did understand, but he just wanted to be a troublemaker (that was Waqlpurga's opinion).
But rest assured, Sirius and Regulus did not get along. When Regulus started crawling, Sirius made a point to step on his fingers whenever possible. When he began walking, Siriius took great joy in pulling rugs out from beneath him. If Regulus wanted a toy, Sirius made sure to get it first, then not share, or he'd put it within sight, but just out of reach. These actions, of course, made Regulus cry, and Sirius got in trouble for them. The boys soon learned how things worked at 12 Grimmauld Place: All Regulus had to do was run to his mother crying about something Sirius did, and Sirius would be put in his room, leaving Regulus free to play as he pleased; and Sirius learned that he was going to get punished anyway, so he'd better do something mean to Regulus so he'd at least earn the punishment. As the two got older, the tricks became nastier, and the punishments more severe – and Walpurga spent much more time with her friend, Odgen's Old, since the boys grew steadily more irritating.
But before Regulus learned that he could get Sirius grounded at the drop of a hat, he was the victim most of the time. Sirius would lock him up in a trunk, or in the attic, or in the bathroom, or stuff him in the dustbin, or under the bed. Once, Sirius managed to get him in the loo – and flushed. Sirius thought it was very funny, until Walpurga caught him.
But Sirius' finest moment came when he first displayed his magical talents. Sirius was three, and he was thirsty, and he couldn't get his mother's attention for anything. Regulus came round the corner, yelling for Mama, and Walpurga came running, and Regulus got a small children's cup full of juice, while Sirius, who had gotten to the point of shrieking, was simply told to shut up. (Kreacher wasn't required to take orders from the children until they were at least four years old – otherwise he spent lots of time eating dirt and worms.)
Sirius was furious when his mother walked away, since he'd been crying for the last five minutes (which seemed like an eternity). When Regulus refused to let Sirius have a drink from his cup (with a very loud "MINE!", which brought Walpurga hurrying back), Sirius had finally had enough. He screamed, and, to his delight, and Walpurga's horror, Regulus started sprouting antlers out of his head. He began screaming, threw the cup, which Sirius managed to catch, and while Walpurga grabbed her younger son and the Floo Powder and raced off to St. Mungo's, Sirius sat contentedly on the floor, enjoying what was now his juice.