I think it's usually best to think of a Visa as a tax or a tariff. It is something required from certain countries. If you don't pay your tax you cannot get in.

A person can certainly be deemed inadmissible to Canada for other reasons even if they are holding a Visa. Unless this mother-in-law has an unsavory and criminal past, however, I doubt that will be a problem in your situation.

I am not familiar with a "super visa". It is possible additional scrutiny was required to obtain this Visa.