Regulus sets his jaw before Bellatrix can see him and it's good, because she immediately draws him forward next to her. "We'd so love if you would. You see, you can never be what you were--"

There's a pause, and Bellatrix steps forward with a sickening smile. "Crucio." Then she waits - she waits until Sirius is on the ground before stepping closer to him to speak again. "You can never be what you were, but you can be something at least with the proper penance."

Regulus is behind Bellatrix again. He's behind Bellatrix again and his brother is on the ground writhing in pain and he makes a split-second decision. He flicks his wand toward Sirius where Bellatrix can't see but doesn't speak aloud, simply mouths the words Protego Duo. A shielding charm, stronger than Protego but not enough for something like Crucio. It's not something visible or noticeable though; no spell bounces off a shining shield, no visible or audible trace of it exists.

The charm slithers and coils around Sirius instead unseen, subtle and hidden and quiet like Regulus is. It won't protect him utterly, but it will make the pain lessen. It's as much as he can do without getting caught.

He's always been terribly good at both charms and not getting caught, that Regulus Black.