What is the ruling on the money that the uncle will acquire from the inheritance of his sister, who is mentally and psychologically ill, given that he forged documents to seize her right?

The question is unclear, but generally, your grandfather's estate is due to all his heirs according to their prescribed shares, and no heir has the right to unilaterally claim any part of the estate without the consent of the others. If the deceased had a salary or an amount disbursed after his death, and it was deducted from his salary during his lifetime, then it is part of the estate and is to be divided among the heirs. If it was a grant from the state or an entity, then it is divided according to the conditions of that entity. Whoever takes any of this money unlawfully is an oppressor, and what he took is forbidden to him. If a dispute arises among the heirs, the Islamic judiciary is the one to settle it.