What is the ruling on a husband going to visit the family of his wife, Nora, on the night of his wife, Sarah, and having dinner with them, and what problems result from that?

It is not permissible for a husband to visit the family of one of his wives on the night of the other wife, because the essence of the division (of nights) is the night. However, he may go out at the beginning of the night for a need, such as prayer, or visiting the family of the other wife or others, provided that the visit does not extend to the point of depriving the wife whose night it is from a portion of her night, otherwise he must make it up to her. He should also not make this a habit or intentionally display it, so as not to provoke the jealousy of the wife whose night it is and upset her.