My elder sister observes parda (hijab) strictly even from the children of our cousin. Someone told to us that they are mahrams as they are your nephews and Islam has declared them as mahrams, which a have brought up some confusion.
According to history we can find that the children of siblings are mahram and other with whom you don't have blood relation are non-mahram as we can find the example of Hazrat Ali and Bibi Fatimah.
So please tell me am I right that children of your cousins are not your mahram in accordance with hadith and the Holy Quran.