In the very beginning, we are to stress the fact that Islam cares much for the maintenance of the highest level of morality and modesty. Indeed, Muslim men and women are both required to keep their sense of commitment to Allah and preserve their chastity. To achieve this noble goal Islam commands Muslim women to put on Hijab so as to preserve their modesty from the hungry eyes of those who have nothing to do with piety.
Coming to this issue, a woman is allowable to put off her Hijab before her nephews; yet she is to wear modest clothes in front of them. Brothers’ and sisters’ sons of a lady are among her maharim (unmarriageable relatives). This is based on Allah’s saying: ” … and not to reveal their adornment save to their own husbands or fathers or husbands’ fathers, or their sons or their husbands’ sons, or their brothers or their brothers’ sons or sisters’ sons…” (Surah Al-Nur 24:31). It is worthy of mentioning here that this ruling is applicable to “nephews” who are the brothers’ sons or the sisters’ sons, not the brothers of one’s spouse. The later is dealt with as other marriageable men.
The late Sheikh Ibn al-‘Uthaymeen, a prominent Saudi scholar, said in his book Majmu`at As’ilah Tahumm al-Usrah al-Muslimah, “Allah says concerning those to whom a woman is allowed to reveal her adornments “…or children who know naught of women’s nakedness.” (Surah Al-Nur 24:31). Hence, if a child shows some awareness of a woman’s `awrah (body parts that should be covered in public) and starts to look at her, then it is not permissible for a woman to uncover in front of him. This varies from one boy to another in terms of natural disposition and in terms of the company that he befriends. The basic rule is that which Allah has set in His saying: “…or children who know naught of women’s nakedness.”
Undoubtedly, today movies have caused children to be aware of women’s ‘awrah at an early age, so we have to be very careful in this regard.