Sirius was completely dumbfounded at how Bellatrix even knew that this was Remus unless she had been following them. He didn't think that could be possible. Remus would have picked up on something much sooner.
He sneered at Bellatrix and her smart mouthed reply. Did she think this was a game? She didn't know a damn thing about what could be going on. What if Severus' potion didn't work just how Remus thought it did? What if something had gone wrong? He didn't think his cousin stupid but he was seriously questioning her competence now.
He looked over to the young man with her. More question running through his mind "And who the hell are you?"