Is it permissible for a woman to travel for Umrah accompanied by her niece’s daughter and her husband, and is her niece’s daughter’s husband considered a mahram to her?

A woman is not permitted to travel without a mahram. Her niece's husband is not a mahram to her. However, if the Umrah is Umrat al-Islam (the obligatory Umrah), then she is permitted to travel with trustworthy company according to the Malikis and Shafi'is. Sheikh al-Islam Ibn Taymiyyah held the view that a woman is permitted to travel with trustworthy company or reliable women if the journey is for an act of devotion, even if the Umrah is not obligatory.