I've never heard of being invited to a party then being expected to pay for it. I don;t care if it is at a restaurant or someone's house. It is tacky! I can see if it were a work party where everyone decides to go to a restaurant and treat the co worker to a party. Your mother should have picked up the tab for everyone. I hope you told her that when the checks came and that no one had to actually pay for a party they were invited to. I would have told the host, since it was my mother, right then and there that she was going to have to pay since she had the party.