Is the impermissibility of marriage between a man and his paternal aunt or maternal aunt or their daughters, for his brother’s or sister’s son, the same as the impermissibility of marriage between him and the daughter of a strange neighbor?

A mahram is a woman whom a man is permanently forbidden to marry due to a permissible reason. Imam Nawawi, may Allah have mercy on him, said: "A mahram among women—whom it is permissible to look at, be in seclusion with, and travel with—is everyone whose marriage is permanently forbidden due to a permissible reason for her sanctity." Excluded from this are the wife's sister, her paternal aunt, her maternal aunt, the mother of a woman with whom intercourse occurred due to a misconception (shubha), her daughter, and the woman involved in li'an (mutual imprecation). Anyone who does not fit this description is not a mahram to a man, and thus it is forbidden for him to be in seclusion with her or to look at her, whether she is a close relative or not.