Dear XXXXXXXXXXX - It is, of course, up to the lender whether they will accept a mortgage on a property since it is a private contract. There would be no law against borrowing money on the house. The problem is that most lenders would want anyone with an ownership interest in the property to sign for the mortgage.
So the answer to your question is "yes" you are legally able to borrow the money but I doubt that any lender will make the loan without her signature. If she is mentally incompetent then you would either need a power of attorney signed by her when she was mentally competent or you could apply for a guardian ship in order to manage her affairs, including selling her interest in the property.
David Kennett - JD - Attorney at Law
Attorney
25 years practicing attorney