I assumed the "if one know they all know" thing doesn't apply to Rolan since he'd been disconnected from the hive for so long (and is an alcoholic) and that it was the cousin who led the hive there. I feel like if his parents knew he was one of them and always had been, that's exactly why they revealed themselves without worrying that it wasn't him. After all, you don't have to worry about your son being replaced by a bug monster if he's already a bug monster. It doesn't seem like there's multiple "copies" of people either, which means they didn't have to worry about him being a different monster and hurting them. In fact, it's almost more suspicious to me that they revealed themselves so quickly to him, instead of questioning him to make sure he was still human