"I don't remember everything," Regulus admitted. But he did remember enough and he didn't want Sirius to think that he hadn't meant what he had said at the end. He knew that they had fought a lot, but he also knew that they had come to some kind of agreement, and he didn't want that all to be taken away by virtue of the fact that he had been drunk.
"But I still want us to try and be better brothers," Regulus pressed. He was a little afraid that Sirius was going to reject that notion, to try and play it off as it wasn't of importance. This meant everything to him at the moment, and he was certain that his fear and worry was palpable in his voice. It had taken him a lot to work up to bringing this up to Sirius again.