Is the mother sinful or negligent in the death of her mentally challenged daughter who took an overdose of the mother's sedatives while she was asleep, and if she is sinful and unable to fast due to her old age, what is incumbent upon her?

If the mother did not fall short in keeping the medication out of reach of her mentally disabled daughter, then there is nothing upon her and no expiation is necessary for her.

However, if she did fall short or was negligent in keeping the medication, then she is liable for the expiation of accidental killing: freeing a believing slave. If she cannot find one, then fasting for two consecutive months. She must also repent, and her 'Aqilah (male relatives on the father's side) owe the blood money to the heirs of the deceased if they demand it.

If the woman is unable to free a slave or fast due to her old age, then there is nothing upon her and she is excused until she is able to do so. Feeding the poor is not a substitute for fasting in this case.