Is it permissible to travel with one's sister and her husband on the basis that he is a temporary mahram?

It is not permissible for a woman to travel except with a mahram, due to the Prophet's (peace and blessings of Allah be upon him) saying: "A woman should not travel except with a dhī mahram (a male whom she is permanently prohibited from marrying)..." A mahram is someone whom a woman is permanently forbidden to marry. Therefore, a sister's husband cannot be a mahram.