Is it obligatory for my mother to make up for the missed Ramadan fasts, knowing that she did not fast them out of fear of my grandmother, and she is now 60 years old and suffers from a chronic illness, and has performed Hajj several times?

Your mother erred by breaking her fast in Ramadan without a valid excuse, or by delaying making up the fasts for years. She must repent and make up the days she missed, as this is a debt upon her, for the Prophet, peace and blessings be upon him, said: "Allah's debt is more worthy of being paid off." She is not required to pay an expiation for delaying the makeup fasts if she was ignorant of the prohibition of delay. Her obligation to make up the fasts is not waived simply because she performed Hajj more than once. If she is unable to make up the fasts due to old age or an illness from which recovery is not expected, then she must feed one poor person for each day (approximately 750 grams). Muslims in general are advised to acquire knowledge and understand their religion.