Did I err in obeying my mother when she prevented me from caring for my sister's children, and singled out me and my children with some food to the exclusion of others, which affected my niece?

It is not in accordance with the ethics of Islam for a mother to eat the most delicious food while her hungry grandchildren look on, nor to treat them with harshness and coldness.

The questioner has two duties: 1. Filial piety towards the mother (Birr al-Walidain): This is the most emphasized of duties, and it carries a great reward for the dutiful and a severe punishment for the disobedient, as the Prophet (peace be upon him) said: "May his nose be rubbed in the dust, may his nose be rubbed in the dust, may his nose be rubbed in the dust, the one who reaches old age with his parents, one or both of them, and does not enter Paradise." And the Prophet (peace be upon him) said: "The greatest of major sins are... associating partners with Allah, and disobeying one's parents." 2. Fulfilling the trust in caring for the sister's children: It is not permissible to leave them to harshness and deprivation.

Filial piety towards the mother does not contradict fulfilling the trust; if you fulfill your mother's rights, she does not have the right to prevent you from fulfilling your other duties, as the Prophet (peace be upon him) said: "There is no obedience to a creature in disobedience [to Allah]. Obedience is only in what is right and lawful." You should treat these children with kindness, making up for any previous shortcomings.