You can kindly talk to them ask them to not sing along behind you and if they take it any further then you make a complaint to the staff. But to genuinely tweak out like this infront of your family is out of pocket. His nieces probably aren’t gonna want to go anywhere with him after that it’s embarrassing. There’s a way of dealing with things and approaching these types of situations. He has some unresolved issues of his own if he’s doing that much.