Bellatrix sent a powerful blast straight at Sirius, which didn't touch him, but threw him off his feet into the wall at his back. How she loved using the magic her beloved Dark Lord gave back to her!
"Careful, little cousin! I wouldn't want to see you accidentally stumble back into oblivion. Not when i can send you there again myself!"
Her cousin would man an excellent sacrifice to her Lord, even if he was recycled goods!