Regulus's whole body tensed when his brother stepped forward and reached for him, having thought his brother was going to strike him. Did he really mistrust his brother that much? Or was it residual from the scoldings he'd received from their mother?
But when he realized his brother was only going to touch his arm -softly at that- he relaxed, at least a little. He was quiet for a long moment, his eyes searching Sirius's face for... something. He wasn't sure what. And he had no idea what to say.
After a moment's more time, what came out was only, "Okay."