My wife provide health care to her Mom (86). Does she get pay?

missing, there is no back pay unless the family agrees to it or it is written in the Will. Many cultures and families see this as a duty, not something they get paid for.

There is no law saying that a grown child has to be paid a salary for caring for their own parent [or grandparents or aunt/uncle or spouse or child].

In fact, majority of grown children are not paid for being a caregiver unless the parent can pay from their own retirement fund. If your wife's mother was able to do this, your wife would have needed to have put together an employment agreement, but that cannot be done after the fact.