Is the deceased's nephew, whom my father sponsored, entitled to claim inheritance from my grandfather's land, as well as from the plot of land my father bought that was in his name, knowing that my father has passed away and we do not know if his father left any provisions with our father?

What your grandfather left is to be divided among his five sons. One-fifth of the land belongs to your father's nephew, as it is his father's share. If the son is his father's sole heir, he takes the entire fifth; otherwise, it is divided between him and the rest of his father's heirs. As for the plot of land your father bought with his own money, his nephew has no right to it, because guardianship is not a cause for inheritance. However, if your father bought the land with his nephew's money, or made him a partner in it, then he is entitled to a share commensurate with the amount of money he invested in it, and you must give him his right.