Can the husband's father revoke the mother's guardianship over her two daughters, especially since he is threatening to do so after the mother refused to give him a general power of attorney, and he has seized the deceased's car and has not given the daughters their share of it? And does he have the right to seize their money?

If the sister has a document proving her guardianship over her two daughters from their father, then she has the right of guardianship and the right to manage their money for their benefit. The grandfather does not have the right to revoke her guardianship or to be their guardian. The sister is entitled to her share of her husband's inheritance, and the grandfather is not permitted to take anything from it without her consent, nor is he permitted to encroach upon the shares of the two daughters. Allah, the Almighty, says: "Indeed, those who devour the property of orphans unjustly are only consuming into their bellies fire. And they will be ˹certainly˺ burned in a blazing Fire." This matter requires taking it to a Sharia (definition) court. The sister has the right to custody of her two daughters unless something occurs that abrogates it.