Chris wanted nothing more than to shove Isaac away and pull Allison to his chest. He wanted to hold her and never let her go. But he understood young love, and Isaac was the one she'd been drawn to when she was on the other side.
He wanted to give them their time together. He knew they were both emotionally raw, just as he was. He was the adult, though, and he would stay back, for now. Mostly.
He moved in and put his arms around the both of them. He kissed Allison's temple, and patted Isaac's shoulder. "Come on. Isaac. Take Allison to the bedroom. Derek and I will get this place cleaned up," and presumably Malia would take care of Stiles.