Is it permissible to give the salary of a mentally ill sister to a poor brother, while providing for all of her needs?

It is not permissible to dispose of the sister's money except in ways that are in her best interest, and it is not permissible to give it to the brother, regardless of the circumstances. Whoever is unable to manage their own money due to foolishness or insanity, their case must be brought before the Sharia court to have them interdicted and a guardian appointed over their money in a way that serves their best interest. Before interdiction, no one has the right to dispose of their money except for their maintenance and necessities.

The guardian must preserve and develop the interdicted person's money, spend on them reasonably, and on whomever they are obligated to support, and pay the Zakat on their wealth.

Interdiction and its consequences are serious and complex matters, and they must be referred to the Sharia courts to investigate their causes and to select a suitable person to be the guardian over the interdicted person's money by judicial ruling.