Well where my partner is from, Newfoundland, I know that a brother-in-law or sister-in-law can sponsor you in the provincial nominee program but I don't know if that applies to mother-in-laws.
But I agree with everyone else, I think it would be more of the responsibility of your wife to sponsor her own mother seeing as it is Family Class sponsorship.
If she meets minimum income requirements, she put her mom down on the additional family form for her PR application, and her mom has a medical examine/criminal background done then I don't think there should be a problem. Unless she has convicted any serious crimes within the past few years or has severe medical problems.
However, I could be mistaken and I am sorry if I am. So senior members please feel free to correct me if I am wrong on anything.
