What should the heirs lawfully do regarding their grandfather's siblings from whom inheritance was withheld because their father adopted a male child and attributed him to himself?

Adoption is forbidden in Islam, and no legal rulings, such as inheritance, are based on it. Therefore, whatever the adopted son took from the inheritance is not rightfully his, and he must return it to be divided among the legal heirs, unless the deceased had willed it to him. In that case, he may take it up to one-third of the inheritance, and anything exceeding that depends on the permission of the heirs. If the money is not returned to the estate, then whoever took something in excess of their legal share must return the surplus to its rightful owners.