My grandma just moved out of her farm and we are trying to...

My grandma just moved out of her farm and we are trying to buy it. She is willing to hold the mortgage for us but our worry is that when she someday passes away will the mortgage have to be paid out in full to whoever is entitled to her estate or is there a way to make a legal agreement so that part of her estate would be prolonged and we would be able to continue to pay out the mortgage as it was originally set. hope this makes sense, Thanks From Canada