Sirius couldn’t help it, he started laughing. That was a "Regulus Black" move if he ever heard of one. While it was funny, Sirius still felt that pang of heartbreak over Regulus being a death eater. That knowledge was never easy for Sirius to accept. In his eyes, Regulus would always be the cute little smiling boy with a headful of curls. Sirius kept that vision in his head when Regulus grew into a moody teenager who "drank the family koolaid" and it brought him some comfort.