Is it permissible to prevent a wife from visiting her paternal uncle and grandfather due to the uncle's theft from the husband and defamation of his reputation, and the grandfather's injustice towards the husband?

It is not permissible to prevent the wife from visiting her grandfather and uncle, because that leads to her severing ties with her kin, which is obligatory to maintain. The uncle must be advised to remember Allah so that he returns the stolen money, for such an act is injustice, and injustice will be darkness on the Day of Resurrection. He must also be advised regarding his false accusation, with reference to the verse: (And those who harm believing men and believing women for [something] other than what they have earned have certainly borne a slander and a manifest sin) [Al-Ahzab 33:58], and warned against harming Muslims and seeking out their faults, for a believer has greater sanctity with Allah than the Kaaba.