I call my in laws but their first name. My husband does the same .

Their first names.

MIL - Her first name or Mom depending on the situation.
DH calls my parents by their first names.
I call my mom, her nickname. But my dad is always dad.

Even though I am close to my in-laws, I didn’t have a good way to address them until I had kids, when I started calling them by their grandparent names. They don’t want to be called by their first name, but they also seemed to not like Mom/Dad. MIL is from the south and everyone younger calls their elders “Miss Larla” or “Auntie Larla”. They sign their cards to me “Mom and Dad (last name)”.

DH calls my parents grandparent names or sir/ma’am. He’s conditioned from his upbringing to not use their first names, even though they’ve said that’s fine.

I called them Mr. and Mrs. LastName until we got married and my MIL verbally beat that habit our of me lol.

DH and I started dating when we were 18 so it just carried on. From marriage to birth of our kids, I used their first names. Now our kids are young and call them Nana and Papa so I usually do too like "Bye Nana and Papa it was so nice to see you!".

After this phase I may switch to Mom and Dad. I love them dearly.

First names. My DH does the same.