The ruling on a man traveling with his wife's sister while his wife is present

Question:
If my wife's sister, who is a widow, is with me, and I am not her mahram (a relative one may not marry), is it permissible for me to travel with her and my family to a distant place, such as al-Ta'if, for example?

Answer:
If you are her only wali (guardian), then she is with your family, like a servant who has no wali (guardian) who stays with your family and travels with them, and there is no mahram (relative one may not marry) due to dire necessity. However, if she has a mahram (relative one may not marry), she should be with