Killing one's daughter by mistake after dropping a stone on her
It is obvious that the stone fell from your mother on your younger sister by mistake when she was bringing water to you from rain, and this caused the death of your sister. If the case was as mentioned, it would be obligatory on your mother to
offer a Kaffarah (expiation), which is (Part No. 21; Page No. 391) freeing one believing slave. If there were none available, she had to fast two consecutive months since she was the reason behind your sister's death. As your mother died before she offered expiation, the price of freeing one slave must be paid out of her legacy, if she left money. If she does not have money and the believing slave is available, one of her sons or relatives may donate the price of freeing him or fast two consecutive months on her behalf. Indeed, he will be rewarded from Allah (Exalted be He) for doing so. That is because this Kaffarah is a debt on your mother that should be settled.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.