LAer – I don;t think she was comparing. It may have been a question as to why she can’t call her MIL, mommy. I think out of respect for the MIL and her husband, she should address her in a maternal matter. Like I wrote earlier, because my SIL is so adamant and for Shalom Bayis purposes, we let the subject drop. It is not worth the fight, considering my SIL will not change her mind about this issue. She has a sister who her calls her FIL, by his first name and feels it is not a problem.
On the other hand, my husband calls MY father by his first name, because my father insisted on it. He did not want my husband calling him Dad, even though my husband does feel funny(about calling my father by his first name) about it. My husband has always been respectful to my father. He kisses his hand when ever we visit.
To the question of what my SIL called her MIL, before the grandkids arrival, she didn’t address my mother as anything. She would just ask her things with out calling her anything. As far as the relationship, between them, it has had it’s moments, but otherwise it’s an okay relationship.
If I did have a MIL, I would have loved to call her Mom. When, my FIL A”H was alive, I called him Dad.