Is it permissible for the questioner's daughters to perform Umrah if they travel with her and her brother's children to perform Umrah?

A woman may travel for Umrah with her brother's sons because they are her mahrams. However, it is not permissible for her adult daughters to travel without a mahram, due to the generality of the hadiths that stipulate a mahram for a woman's travel, whether for Hajj, Umrah, or other purposes. Therefore, whoever travels without a mahram is sinful.