Is it permissible to buy land from an agent of the mentally ill, unmarried, retarded paternal uncle, knowing that the uncle has brothers and sisters from his mother, and is there any sin in that?

If a person is mentally incapacitated, they are treated as a discerning minor, and religious obligations are lifted from them. If the brother’s guardianship over the property of this person has not been established by an authorized body, then the brother does not have the right to sell his uncle's land. Rather, this right belongs to the ruler or the Muslim community in the absence of a just ruler.