Should I fast on behalf of my deceased mother who owed missed Ramadan fasts, or should I offer expiation? If so, what is its amount? And should I fast alone or with my siblings?

He who dies and owes fasts from Ramadan falls into two categories: The first is that he dies before it was possible for him to fast. In this case, according to the majority of scholars, nothing is due from him. The second is that he dies after it was possible for him to make up the fasts. In this case, it is obligatory to feed one poor person for each day he missed. This is the view of the majority of scholars. The amount to be fed is 750 grams of wheat, rice, or similar staples.