Is the mechanism for handling the funds of a sick, incapacitated brother, which his family has established (spending on the brother and his daughters, then on needy family members, and donating to the poor and charitable projects), consistent with the objectives of Islamic Sharia?

It is not permissible to spend from the wealth of an interdicted person except on himself and on those whose maintenance is obligatory upon him, such as his wife. As for his married daughters, their maintenance is the responsibility of their husbands. It is not permissible for the guardian of an interdicted person to dispose of his wealth except in a way that benefits him. Therefore, he is not allowed to give charity or gifts from it, even if the interdicted person requests it, because he is not qualified to make donations.