What do you call your in-laws?

I'm not married yet but I call my fiances parents by their first names if I need to address them directly. His brother in law also does the same. I only call my parents mom and dad.. I even call my step mom who has been in my life since I was 4 by her first name.

I call them by their names and he does my parents as well.

I agree with others who have suggested to do what feels natural to you. If you don't feel comfortable calling your MIL/FIL mom and dad then don't. I personally would be offended if I was asked to do that, because like you, I have my own mom and dad.

I call my MIL/FIL by their first names, which is the same as I did before my husband and I were married. We have a dog and both of our parents are grandparents to our dog. To our dog, my MIL is "grandma that buys you all the treats and toys" and my mom is "grandma that has the little shy dog and the big backyard you can play in" lol.

I call my fiance's dad and his stepmom by their first names. For his mom, I think she's used to being called "Mom" because that's what her son-in-laws call her, but I would never call her "Mom". I prefer calling her by her first name; however, we have been emailing each other (we're big email people), and she mentioned (out of the blue!) that she understands I might feel awkward calling her "Mom" since I have a mom (which is true). So she wants me to call her "Nana", since her grandkids call her that, and "that's what [she] like[s] to be called by everyone anyway". I really would much prefer to still call her by her first name, but I have no problem calling her "Nana" either, I guess lol.