Should the sons of an old divorced woman, who cannot observe Sawm feed the poor as expiation on her behalf?

Q: Since my mother cannot endure Sawm (Fast) due to her permanent feebleness, she has not been observing Sawm during Ramadan for the past twenty years. During this period, she would feed a poor person as expiation for each day wherein she broke her Sawm. Three years ago, she was divorced by my father. Now she is under the sponsorship of both me and my brother who does not live with us in the same house. As we have limited income, is it obligatory on her to feed the poor as expiation, or should we do this on her behalf? It should be known that she has no financial resources. What is the amount of food that should be given to a poor person as expiation for each day? Should the amount of Zakat-ul-Fitr (obligatory charity paid before the Festival of Breaking the Fast) that is due on her be paid by all of us, or does it suffice that one of us pays it on her behalf?

A: You should feed, on behalf of your mother who cannot endure Sawm permanently, (Part No. 9; Page No. 81) a poor person an amount of one and a half kilograms of the prevailing food of the country as expiation for each day wherein she broke her Sawm. It suffices that this be done by any of you. The same applies to Zakat-ul-Fitr, i.e., it suffices that any of you should pay it on her behalf.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.