Not making up for missed days of Sawm till death

Question: My grandmother died while still had to make up for some missed days of Sawm (Fast) in Ramadaan. However, she did not know exactly how many days she missed. She would break Sawm when she was menstruating, during her postpartum period, or when she was extremely tired. But, she did not make up for these missed days out of ignorance. By the time, she had learned the rulings of the Sawm she was already very old and unable to observe Sawm. She died (may Allaah have mercy on her) while still did not make up for these missed days. My mother does not know how many days her mother did not observe Sawm. Will it benefit my grandmother and make up for her missed days of Sawm that my mother observes Sawm on her behalf during the White Days (13th, 14th, and 15th of every Hijri month), along with Mondays and Thursdays, with the intention to dedicate the reward to her mother? Or will it benefit her if my mother makes an estimation of these days and offers Kaffaarah (expiation) for them along with observing Sawm?

Answer: If you are most certain that your grandmother did not make up for her missed Sawm, andshe told you so when she became old, then it is permissible for you to observe Sawm on her behalf with the intention of making up for her missed days of Sawm… read more here.