Regulus glanced sharply at Barty, abruptly realizing he wasn't the only shopkeeper in Diagon Alley who knew about Barty's unsavory past.
"Yes, he is Severus' youngest brother. Severus has a shop here in Diagon Alley, too: Alchemy and Ecstasy. Gaius works there along with their other brother, Serenus." He paused, then added casually, "I will let Severus know you are working here. I imagine he will be interested to have word of an old schoolmate."
Hopefully, Barty would understand Regulus meant he would give Severus forewarning that Barty was working there and not up to any mischief.