What is the ruling on the action of a maternal uncle who seized his sick sister's share of the inheritance for himself, claiming to be her guardian, and refused to spend it on her or allow anyone else to do so, and threatened anyone who objected to his behavior? And what is the way to deal with him to recover the aunt's right?

A brother has no guardianship over his sister's money unless he is a legal guardian appointed by the father, grandfather, or judge. He is not permitted to take her money or dispose of it, especially if he spends it for his own personal interests. The matter must be referred to a judge to appoint a trustworthy guardian over the woman's money, who will manage it in a way that serves her best interest, including her medical treatment if needed.