Is it permissible for the maternal uncle to spend from zakat funds on his niece—who suffers from mental retardation and was abandoned by her father—to cover her needs for hygiene products and clothes, knowing that the father will inherit her if she dies? What is the appropriate action in this case? And is the questioner sinful if she financially sponsors the girl despite her dislike for her due to her behaviors?

It is not permissible to give the daughter from Zakat (definition) as long as she has sufficient money, unless she is one of the eight categories eligible for Zakat. If charity is given to her, that is good and an act of kindness. If the father falls short in spending on his daughter, this does not prevent him from his right to inherit from her. The father should be advised and reminded of his duty towards his children. Whoever cares for the daughter and treats her well, seeking to please Allah and her husband, will be rewarded for that. There is no sin upon whoever finds difficulty or dislike in her heart towards the daughter due to her actions, for a person is not held accountable for matters of the heart.