goodness i hate this subject
My father in law, his wife, and my stepfatherinlaw i call by their first names. My father in law says "hi it's dad" to me on the phone or whatever. I don't even call my stepfather dad, and he's more of a father to me than my real dad is.
My mother in law is a different story. I was never "told" i could call her by her first name, and while my wife and i dated i called her Mrs. X. so now that i'm the son in law, i have a tough time calling her by her first name, i won't call her mom, and i can't call her mrs. X.
to compound it, my mother and my mother in law have the same first name. so... now i'm really messed up cause.
to compound it further... my sister in law calls EVERYONE of them mom and dad... so i feel weird that i don't