trying to buy a relative out of foreclosure is known as a foreclosure bailout and if a lender finds out that you are a relative they will deny the loan. They will look at last names and prior address for a connecton to sniff out a foreclosure bailout. Their thought process is this: you are buying the home but the person (who couldn't nake the payments before) is going to make the payments and they see that as a recipe for disaster.
Try the link below to see if a loan modification is an option. If not talk to the bank that holds your mortgage about a modification and adding a co-borrower. That may work better than tryig to buy through short sale
Alan Openshaw NMLS # 143960
Cornerstone Lending Inc