"And you've been taking it?!" he asked, his incredulity more powerful than his disappointment at the announcement that Padfoot wouldn't be needed. He loved those monthly romps through the woods, even if Remus didn't, and Sirius felt their loss acutely. He'd missed them, but this was loads more important: sure, Regulus seemed to want to present himself as some sort of hero, but Sirius wasn't fooled, and he'd wanted to imagine that no one else was either, but here was Remus, willingly taking a potion he didn't know from him.
"You know he's a Death Eater, don't you?" he asked, in the same harsh tone. "You haven't forgotten? My mother's son? That Regulus?"