MawtheMerrier wrote On your own admission ...
No, I stated it, not admitted it.
The fact of the matter is that one lady was upset because her daughter-in-law never addressed her by anything.
> A mother in law is not an Aunt, so nobody in their right mind refers to her as such.
A child addressing a female friend of his or her mother as Auntie is not uncommon, so a person addressing his or her mother-in-law as Auntie is in much the same style, in fact more appropriate.
How should a woman address her mother-in-law? As Mrs Smith, which is after marrage quite possibly how she herself is known to people,? As mum, if she doesn't feel right about it as she has her own mother? As Jane, if she feels that the family tree generational difference makes her feel awkward about it? Not address her as anything - and risk her mother-in-law feling miffed? Or address her, if so invited, as Auntie Jane, warmly and lovingly, while respecting the generational difference within the structure of the family?
The bit about "nobody in their right mind" is just you saying that. If the Auntie Jane format works in some particular family then, in my opinion, that is fine and a perfectly sane option. It might be just the thing so that the mother-in-law knows that she is regarded as if an aunt, close but not too close.