I never called my mil anything for a lot of years,until I got fed up with her bossy ways and started calling her,her first name it seemed to put us on a more equal footing, the children called her grandma.
My SIL's have never been invited to but have always used my first name which I don't mind,I suggested my daughters call their Mil by their first names from the off they were much older than I was when I first met mine, it seems to work.
One grandchild calls me grandma the other granny and my DH grandpops or grandpa. They have lived abroad and call all adults aunt and uncle. I was made to do the same when I was small even my mother's friends.
When I was small we called our grandparents mother and father which they weren't but was a local custom even shortened by some to mud and fad. I think my father must have had trouble with my gran because he called her Mrs...... my uncle called her mam.