I always thought that you should address the FI's paretns as Mr. & Mrs. until you are married (or ask them what they would like to be called). Some like to be called Mom or Dad, or by their first names. I have no problem with that.
Someone forgot to tell my brother it's ok for my BIL to call them Mom and Dad!
He had a total fit last night and said he'd kill my BIL and my BF if he heard them call them Mom and Dad!
Excuse me, but I thought that was something between the future/current in-laws and my parents. Since when should my bro have a say?
I have to laugh only because my BF is quite a bit bigger than bro, yet bro said he'd take my BF and BIL on at the same time!