He involuntarily flinched when she got that look in her eyes. That was the scary Lily that Sirius had come to know. It wasn't often that he saw her in this way. Which made it all the more terrifying. Sirius had never met Lily's sister. He had heard stories though and he could understand Lily's response. It was about the same way that he was with Regulus.
"I can only assume it was Dumbledore," he replied. In truth, Sirius didn't know very well. He had gone after Peter right after Harry had been taken from him and from there he had gone directly to Azkaban.
"Believe me, Lils, if I were a free man, I would have raised Harry happily."