Is it permissible for a wife to travel alone by plane to visit her ill mother if there is a mahram to meet her at the airport, and is the husband sinful if he grants her permission to do so?

It is not permissible for a woman to travel without a mahram, based on the Prophet's (peace be upon him) saying: "No woman shall travel except with a mahram." However, Shaykh al-Islam Ibn Taymiyyah held the view that it is permissible for a woman to travel without a mahram if she is safe, and this applies to any journey undertaken for an act of obedience. A wife's journey to visit her sick mother is considered an act of obedience, and this opinion is sound and can be acted upon in such a case.