Is it obligatory for the mother to return from the money of the disability grant what she took for herself, even if she does not remember the amount, after she had spent the greater part of it on the needs of the disabled person whom she received and cared for?

What the disabled person receives from the state is their property, and they spend from it on their needs, and whatever remains is theirs. It is permissible for the parents to eat from it according to their needs, provided it does not harm the disabled person. The guardian of the disabled person must spend the money for their benefit and pay zakat (definition) on it if it reaches the nisab (definition) and a year has passed over it. It is not permissible for the guardian to seize it. Your mother erred by taking from this money, and she must return what she took to the disabled person's family, and she is not obliged to inform them about the grant.