You may not and cannot buy a house for a loss, or below what the Mortgagee owes on it.
Your Lender will not let you; because someone has to make up the difference, somewhere.
People have been trying to circumvent this for years.
You should pay fair market value for the property even though you are buying from a family member. At closing, part of the money you are paying will go to pay the mortgage and back taxes instead of going to your relative.
Licensed Real Estate Associate Broker
450 7th Avenue Suite 1501
New York, NY 10123