Old Master Xiao listened to Situ Xin's words and did not reply immediately. Instead, he pondered for a while before lifting his head to look at Situ Xin and said, "Xiao Xin, I'm entrusting Muli to you." Old Master Xiao's statement signified his complete trust in Situ Xin.
Old Master Xiao had his own thoughts as well. He had watched Situ Xin grow up from childhood and knew her character well. She wouldn't undertake something unless she was sure of it. Another more important reason why Old Master Xiao agreed to let Situ Xin take action was her affection for Xiao Muli. Without necessarily being love, there was at least familial affection from Situ Xin towards Xiao Muli. On this count alone, Old Master Xiao knew Situ Xin would not let anything happen to Xiao Muli.