Regulus had stuck close to Sirius for two reasons. One, he didn't want to be left alone in the middle of this chaos, and two, he had a feeling Sirius would get fixated on Bellatrix and would need someone to watch his back.
Muttering about the predictable foolhardiness of Gryffindors, Regulus sprinted after Sirius, catching up just in time to see him topple to the ground.
"Stupefy!" he shouted, immobilizing the Death Eater - one who must have been after his time because he didn't recognize the face - before she could do anything else to Sirius. He bound her and levitated her over to the wall, and then he went to Sirius.
"If you're lucky, I might let you forget this before we're in our dotage," he said. "Finite Incantatem!"