Note: This is not about your mother in law, my mother in law, or anyone else's mother in law. It is about the stereotypical mother in law.
If someone is spending Pesach with his mother in law then you just wish him a chag kasher because it probably won't be sameach.
If he is spending Pesach at a hotel you just wish him a chag sameach because there might be problems with the kashrus.
If he is spending Pesach with his mother in law at a hotel then you wish him a "chag".
