Islam says to respect your mother in any case, but does not allow you to oppress your wife in doing so. Respect is something other than to listen to her even when she is directing you to something wrong. In your case, you must assist the Peron who is "right" be it your mother or wife. But in any case you must not disrespect or belittle your mother.
No mother is happy to see her children unhappy..in this case it looks like your mother feels neglected or left out. To mend this I would suggest you spend some solo time with her and give her attention. She needs to feel.more.loved and cared for. All this should be done minus your wife..just you and your mother.