There is no sin on them [the wives of the Holy Prophet صلی اللہ علیہ وسلم] with regard to their fathers, their sons, their brothers, the sons of their brothers, the sons of their sisters, their fellow women and their slave women. They should fear Allah. Indeed Allah is witness over everything. (Surah Al-Ahzab: 55)
"RuhulMa'ani" reports that when the verse of Hijab (veil) was revealed, the relatives of the Holy Prophet's صلی اللہ علیہ وسلم wives began to ask whether the laws of Hijab (veil) must also be exercised with their family and relatives. It was then that the above verse was revealed, permitting women to be seen by the men mentioned in the verse only.
Al though the verse cites only four categories of men, others are also regarded as Mahram when they have a close blood relation with women. The maternal and paternal uncles are not mentioned in the verse because they are just like the father to a woman.