I called them by their Christian names. It was the only thing I felt comfortable with,after all,My mum died when I was 17,and my dad is still alive at 84.I could never imagine calling anyone else Mum or Dad,not even my beloved Step Mum who I also call by her Christian name.
I couldn't imagine calling my MIL mum, it just seemed too weird so I call her by her christian name. My husband does the same for my mother. My mum had a row with me about it though .... asked what was wrong with me and why did I always have to be different to other people. She was stuck in the 1960's :)
Our surname is Harding and both of our daughter in laws call us Mr H and Mrs H. They have been in our lives for many years before marrying our boys. Grandkids are a new addition, will be interesting if the girls call us the name the Grandkids choose.
I call them by their first names. They are not my parents, so I couldn't call them mum and dad. When I first met them I called them Mr and Mrs L. After we got engaged, they said it would be ok to call them by their first names and that has suited us after all these years
Sadly, only a few short days after writing my reply, here I am planning dear father in law's funeral as he passed away two days ago. I never would have imagined when I replied to this question that he would be gone so quickly. I am going to miss him so much :((
By their christian names, & that's what my husband called my parents also by their christian names. Our daughters call their inlaws by their christian names as well.1 sister-in-law called my in-laws mum & dad, after her parents died.