Regulus isn't letting go of his hand, but Sirius is letting go of Regulus'. He can't help it. He absolutely has to move in to hug his brother, no matter how public a place they're in. It's an instinct older than everything they've been through.

"Don't apologize," he says fiercely, "You think I don't realize you almost got killed trying to save my life? Merlin, Reg, she was trying to get me to hurt you. Why didn't you just play along?"