Sirius went with the rest of the Order to the Ministry and ended up dueling his cousin Bellatrix Lestrange, who had escaped with a bunch of other Death Eaters from Azkaban earlier in the year. Of course, the duel was all for show—the two Blacks had ended up in cells opposite each other in the prison and had become rather close (or as close as they could, when one was clinically insane and the other was essentially a man-child). It was all fun and games until they ended up in front of the Veil.