Are the actions of a wife who stays up late with her male cousin and kisses him, claiming he is like her brother, Islamically permissible?

Islam has regulated the interaction of a man with a non-mahram woman. It has commanded lowering the gaze, forbidden khalwa (private seclusion), prohibited shaking hands, enjoined covering the body, and prevented softening the speech. Your wife has transgressed these boundaries by kissing her cousin, staying up late with him alone, and not wearing the hijab in front of him. The leniency on the pretext that the cousin is like a brother is a grave mistake, for the cousin is a non-mahram, and the harm from him can be more severe due to the laxity in dealing with him. The in-law (relatives of the husband other than fathers and sons, such as a brother or a cousin) is death, meaning that the fear from him and the temptation are greater. It is obligatory to clarify the boundaries of what is lawful and unlawful to your wife, and to prevent this leniency, for you are responsible for your flock and protecting them from the Fire.