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I am sorry to learn of this situation.
If your wife is incapacitated, her condition is going to be controlled by either an advance directive (such as a "Durable Power of Attorney" or simply an "Advanced Directive"), which is a document that she would have signed before she became incapacitated, and designates an individual (or individuals) to make medical decisions on her behalf.
If this did not happen, you can petition to be your wife's guardian - this is a court process, it is not terribly complicated, but you do need to go through the process to get legal authority to make healthcare decisions for your wife. I would recommend visiting your court's self help desk (many states have online court self help documents to assist in a guardianship/conservatorship application).
It is unusual that they would be speaking with your wife's mother and not you - but this does lead me to believe that perhaps there is an advance directive on file which gave this authority to your mother in law. Speak with the hospital first and see what the current legal status is before spending time pursuing a guardianship (at least that way you will know what you are working with).
If the hospital is simply acting without any legal documents - speak to the Hospital's Ombudsman, they should be able to assist you in recalling the clinical staff to their proper duty (but still pursue the guardianship).