All my DILs use my first name. I simply let them choose whatever form of address they felt comfortable with, I had no preference. Cards sent to our DILs are signed with our first names, but if it's to them as a couple we sign 'Mum and Dad'. We also use nicknames a lot in our family so I might call them Twinkle Toes, Lofty or something else occasionally the same as we do with our sons. The important thing is that we are all happy with it and get along very well.
I never really called my own MIL anything. A few weeks after we married I answered the phone to her and without thinking said "Hello Mum.... etc" There was silence then she said very seriously "I'm not your Mum, I only have one child. Is he there?" Her feelings on the matter were very clear. When our children came along they called her Nanny, so I did the same, she didn't seem to mind. Although I liked her she never seemed to relax with me and we were never close. She simply couldn't get over the fact that I had 'stolen' her beloved son from her. Shortly before she died she remarked that I was a good wife and mother but she wished I had married someone else!! She also said our children had never really liked her, which was not true, they just found her rather formal and difficult to talk to, and they didn't fit into her idea of adoring seen-and-not-heard grandchildren. Generational differences I suppose. But now, as adults, they remember her warmly.