Her estate is responsible for the mortgage. You have to pay the mortgage out of whatever resources her estate has or “sell” the house to the grandkids at which point they are responsible for the mortgage. The bank has to get their money somehow. If the grandkids don’t want to assume the mortgage and the estate doesn’t have the money to pay the mortgage then you will default and the bank will foreclose and take the house to auction.
I would not let the house go to auction. That will be the least advantageous. Having her sell now means that she has to have the balance of the mortgage available at closing to pay down the loan. That would of course reduce the value of her estate and then it’s less going to the grandkids or whoever. Whether she or her estate sells when she is alive or afterward is likely moot. She still owes the bank the balance of the mortgage no matter what.