Is it permissible to use the testimony of a non-upright relative in a marriage contract, and does the marriage become void with such testimony?

It is not a condition for the witnesses of marriage to be relatives or acquaintances. Rather, it is sufficient for the contract to be witnessed by two just Muslim men. Knowledge of the true nature of justice is not a condition; it suffices that no open transgression is apparent. If the transgression of a witness becomes clear after the contract, it does not affect the validity of the contract. As for the testimony of a transgressor regarding the validity of marriage, there is a difference of opinion among scholars concerning it.