For a moment, despite Sirius' tone, Regulus thought that Sirius was going to hit him. The sudden blur of motion startled Regulus; he reacted on impulse, taking half a step away from Sirius, trying to figure out how to get out of the other's way if he really was aiming to take a punch. There wasn't a lot of room in the kitchen and while Regulus had years of Quidditch practice to keep him in shape, he didn't doubt that Sirius would be able to easily outstrip him in an actual fight.
His hands had been half up and ended up pinned against his chest when Sirius pulled him into a hug. His body stiffened a little; he didn't know what to do. He hadn't expected this in the slightest. He didn't... didn't really understand what Sirius was doing or what he thought he was doing. For just an instant, Regulus second-guessed whether the person in front of him was really his brother or not. He didn't know who else it could be though. Regardless, it had been years since Sirius had hugged him.
"I've only just arrived here," Regulus answered, his words coming out somewhat strangled because of the tight grip that Sirius had on him.