What must the mother and uncle do regarding the missed fasting days of their deceased grandmother from Ramadan 30 years ago, which the mother suspects were not made up?

If a person breaks their fast in Ramadan due to an excuse, and the excuse persists until their death, then there is nothing upon them. However, if they were able to make up the fasts but did not do so until they died, then the preponderant opinion is that it is permissible for their guardian to fast on their behalf, based on the Prophet's (peace be upon him) saying: "Whoever dies owing fasts, their guardian should fast on their behalf."