Is it permissible for the brother to continue spending from his father's grant after the latter lost consciousness, or must it be divided among the siblings?

If a father loses his mental faculties, guardianship over his wealth falls under the purview of a judge's ruling, and none of his children are permitted to assume sole control over it. A grant (or gift) is not an inheritance, as the father is still alive. The individual appointed by the judge to manage the father's affairs should disburse it for the father's needs and for the sustenance of those he is obligated to support, with any remainder preserved to be managed according to what is deemed beneficial. If the father dies, the remaining amount becomes an inheritance to be distributed among all heirs.