I never had a MIL and as for my FIL he has been gone for so many years now I can't remember what I called him. My husband always called my mom Gram once we had kids. Probably avoided having to call them by a name before that. My DIL (both current and ex) and SIL call us Mr. and/or Mrs. O. Works for us. I've been Mrs. O to so many kids that I have been with in Girl Scouts and as a sponsor with the high school band etc. that it is very comfortable for me. The grandkids all call us grandma/pappy.