I call my MIL by her first name. Although now that we have had kids I tend to call the Nan and Pa half the time as I'm referring to them in front of the kids and its easier.

I was seeing a guy ( long term relationship) and they expected me to call them Mum and Dad, and I honestly just couldn't do it, it didn't feel at all right and must say I got annoyed that they expected me too. Hence to say that relationship didn't last.