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How do you address yours, EP?
Alas I have never been married, so no mother-in-law. but had it happened I think that I would, if she had agreed, have addressed her as Auntie Given-name, as, being both a generational difference and in a family setting, that would have seemed best in my opinion.
Please note I am here expressing my opinion of what I would have done, I am not in any way saying what other people should or should not do.