Is it permissible for the close female friend to be a mahram for the purpose of going out with the fiancé, knowing that she is the fiancé's maternal cousin and the sister of his paternal uncle's wife, and with the presence of the fiancé's sister with them?

Islam stipulates, for a woman's travel to be obligatory, that a mahram be present with her, and the presence of one or more women is not sufficient. As for within the city, it is permissible for a woman to go out with a non-mahram man if seclusion is avoided by the presence of another woman. However, if there is no one else with them, it is forbidden for her to go out with a non-mahram man, even if he is her fiancé.