Is it permissible for me to travel with my husband's brother, my aunt, and her daughter, without my husband being present?

A woman is not permitted to travel except with a mahram (definition), based on the Prophet's (peace and blessings be upon him) saying: "A woman should not travel except with a mahram." The husband's brother is not a mahram; in fact, there is explicit prohibition against being in seclusion with him, as the Prophet (peace and blessings be upon him) said regarding the husband's brother (al-hamu): "Al-hamu is death." A mahram is a man to whom a woman is permanently forbidden in marriage due to lineage or a permissible cause, such as the husband. If the husband is unable to travel, the woman may travel with other mahrams.