The deal you are talking about doing with your father is a sub2 or 'subject to' the existing mortgage. They are not uncommon and are a great strategy for under water properties. Search on here for sub2 and I'm sure you will find plenty of info.
Kenneth Estes is referring to the 'due on sale' clause. Its a clause that if the property is transferred/sold the bank 'may, at their option' call the loan due on sale. I know of many people doing sub2 deals and I don't know of anyone where the bank called the loan due. As the property is $70k underwater, if the bank did elect to call the loan due, there is no way you could refinance it. I'm assuming you don't have the cash to make up the difference. Therefore, the bank would either have to accept a short sale or foreclose. After all the banks have been through, I don't think they want to force foreclosures. Especially if they are still receiving payments. So, look up 'due on sale' clause for more info.
Now what you need to look at is, does the property cash flow? You should research the 50% rule. Basically in the long run, 50% of rents will go to expenses (all expenses, not just PITI). Then subtract debt payment and that equals your cash flow. Many people on here look for an average of $100/door per month, so $200 after expenses and mortgage is paid.
Of the $2000 monthly payment, how much is just the mortgage?
Take $1500+$850 (rents)= $2350 x 50% = $1175 - mortgage = Cash flow. What is that number?
What could you get for the 1/1 if grandma moved out? Re-figure your cash flow with the new numbers. Is it now $200? I know grandma is emotionally tied to the property but if she moves out to a place with the same payment, maybe you get to keep the property in the family for another 30 years.
The good thing about the property now is you are 9 years into the mortgage. With amortization, the longer you own it, the principal pay down increases.
So now you need to look at your goals. What is the goal for owning this property? If you want a long term hold and are willing to wait on good profits, then it may make sense.