Is it permissible for a daughter to go for Umrah with a mahram other than her father?

If your nephew is not a mahram (definition) to your daughter, his presence does not suffice for your daughter's need of a legal mahram. If performing Umrah (definition) is obligatory for your daughter—for instance, if it is her first Umrah and you are accompanied by a trustworthy group—the preponderant opinion is that it is permissible for a woman to travel for obligatory Hajj and Umrah with a trustworthy group, provided that fitna (temptation/strife) is safeguarded against, and provided she is an adult. However, if she is a young girl, there is no harm in her traveling with you without a mahram.