Nah not horrible. He’s horrible, that’s why it’s easy. Once you find out a life-changing betrayal like that, your strongest emotions would take over (anger, disappointment, betrayal, etc). They may feel bad in private, and they did hesitate, it’s not like they didn’t. The part where ML told them to cut off contact or else, they hesitated. But it’s hard to ignore evidence like that and the fact that your own family member was separated from you for years as a scheme.
Also just look at the other comments, they noticed that the family did hesitate, it’s just the fact that their desire to see their long-lost child was stronger than their fcked up fake family member.
It's kinda weird that it took them like 5 seconds to throw away the kid they raised and loved for years. Yeah they found out he was an asshole and evil, but he was their "son" for like 17-20(?) Years! It's kinda horrible how easy they stopped loving him.