Is it permissible for her daughters to do ‘umrah without a mahram?

It is permissible for you to travel for ‘umrah with your brother’s sons because they are mahrams for you. 

With regard to your daughters, if they have reached the age of puberty, then it is not permissible for them to travel without a mahram, because of the report narrated by al-Bukhaari (1729) and Muslim (2391) from Ibn ‘Abbaas (may Allah be pleased with him) who said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “No woman should travel unless she has a mahram with her, and no man should enter upon her unless she has a mahram with her.” A man said: O Messenger of Allah, I want to go out with such-and-such a campaign, and my wife wants to go for Hajj. He said: “Go for Hajj with her.” 

It makes no difference whether a woman wants to travel for Hajj or ‘umrah, or for some other purpose. In all cases it is stipulated that she should have a mahram with her, because of the general meaning of the hadiths and, in fact, because of what is mentioned in this hadith about the woman setting out for Hajj, and the Prophet (blessings and peace of Allah be upon him) instructed her husband to go out with her for Hajj and to forget about jihad. 

If a woman does not have a mahram, ‘umrah and Hajj are not obligatory for her. 

If she goes out without a mahram, she is sinning thereby. 

Please see also the answer to question no. 25841.