Is it permissible to make up the fasting days on behalf of the deceased woman who intended to feed the poor due to her inability, and is expiation (kaffarah) required for every day she missed, and who is most entitled to make them up on her behalf?

If a woman breaks her fast due to pregnancy or breastfeeding, she is only required to make up the missed days (qada'). If your aunt delayed making up the fasts due to an excuse, and then despaired of doing so because of her old age and inability, she must feed one poor person for each day, and her heirs are obligated to pay this from her estate.

However, if she was able to make up the fasts before her old age but failed to do so until another Ramadan began, and then despaired of making them up, she must feed one poor person for each day. Additionally, she must pay an expiation for delaying the qada' without a valid excuse, which is feeding another poor person for each day. This means feeding two poor persons for each day (which is more cautious), or one poor person (another opinion). If she did not do this before she died, it must be paid from her estate. There is no requirement for anyone to fast on her behalf.