Is it permissible to invite the wife's maternal uncle, who denies the authenticity of the Sunnah and breaks his fast after the time of 'Isha' instead of Maghrib, to an iftar at home, or to visit him and eat with him?

Denying the authority of the Sunnah is disbelief (kufr), but declaring a specific person to be a disbeliever requires establishing proof against them. If a person's disbelief is established, then maintaining kinship ties with them is not obligatory, but it is permissible for worldly matters. There is no objection to a wife inviting her disbelieving maternal uncle for iftar, exchanging visits, and eating with him, provided that this is not done out of loyalty and love, but rather for the sake of kinship, to win his heart, and to clarify the truth to him, hoping that he may repent.