My wife wears the Niqaab and she wants to know whether it is permissible to unveil her face in front of my brother and her sister's husband given that the entire family lives in the same house. Does this reduce her reward for wearing the Niqaab? Please note that she always wears it outdoors and in front of strangers.

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

According to the correct opinion of the scholars, it is incumbent on a Muslim woman to cover her entire body, including the face and hands, in front of all non-Mahram men, including her husband's brother and her sister's husband. In fact, the obligation of adhering to the Islamic Hijab in front of the male in-laws and non-Mahram relatives who frequent the house is more emphasized. ‘Uqbah ibn ‘Aamir reported that the Prophet, sallallaahu ‘alayhi wa sallam, said: "Beware of entering the places of women." A man from the Ansar said, "O Messenger of Allaah, what about the Hamw?" The Prophet replied: "The Hamw are death itself." [Al-Bukhari and Muslim] 

The Hamw are all male relatives of the wife's husband who are lawful for her to marry after divorce or the husband's death.

If a woman unveils her face or hands in front of her in-laws, then she is disobeying her Lord and exposing herself to Fitnah (temptation, tribulation).

Allaah Knows best.