Is it permissible for a mother to waive her share of inheritance in favor of the children of her deceased son, and what is the ruling on the deceased's sisters objecting to that on the pretext of the mother's illness?

Illness does not prevent one from disposing of their wealth through a gift as long as the person is of sound mind and mature, and the illness is not a death-bed illness. However, if a person's mind changes due to senility, or if the illness is a death-bed illness, then their disposition is not effective and is considered in the حكم of a will.

A death-bed illness is an illness that medical professionals have determined to have a high mortality rate.

The daughters have the right to ask their mother not to proceed with the gift and to retract it, due to the potential harm that might befall them because of it.