Can she uncover her face in front of her sister’s husband?

Your sister’s husband is a stranger (non-mahram) to you, and you have to cover your face in front of him and not be alone with him. Similarly it is haraam for him to look at you or to be alone with you. Unfortunately people are careless in their homes with regard to the in-laws, even though sharee’ah speaks strongly concerning them more than others, because people mix freely with them in their houses and the family members trust them. 

It was narrated from ‘Uqbah ibn ‘Aamir that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Beware of entering upon women.” A man from among the Ansaar said: “O Messenger of Allaah, what about the in-law?” He said: “The in-law is death.” 

Narrated by al-Bukhaari, 4934; Muslim, 2172. 

The in-law is the husband’s relative. 

We notice here that the Sahaabi wanted to make an exception in the case of the husband’s relatives, but the ruling was re-emphasized in the strongest terms, because no one regards it as strange if he enters the house. 

Al-Nawawi said: 

With regard to the Prophet’s words, “The in-law is death,” what this means is that the fear in his case is greater than in the case of others, and evil is expected on his part and the fitnah (temptation) is greater, because he is able to reach the woman and be alone with her with no one denouncing him, unlike the stranger or non-mahram. What is meant by the in-law here is the husband’s relatives, except for his father/grandfather and sons/grandsons.His father/grandfather and sons/grandsons are mahrams of the wife and it is permissible for them to be alone with her; they are not described as “death”. Rather what is meant here is the brother, nephew, uncle, cousin and others who are not mahrams. People customarily take things lightly with regard to them, so a man will often be alone with his brother’s wife. This is what is described as death and it is more important that he be stopped than a stranger, for the reasons we have mentioned above. What I have mentioned is the correct meaning of the hadeeth… Ibn al-A’raabi said: This is something that the Arabs say, as in the phrase al-asad al-mawt (the lion is death), because meeting it is like dying. Al-Qaadi said: What is meant is that being alone with the in-laws leads to fitnah (temptation) and the destruction of one’s religious commitment, so this is described as being akin to the destruction of death.  

Sharh Muslim, 14/154. 

We advise this sister and others to fear Allaah and strive to wear proper hijaab in front of non-mahram men. 

Please see also questions no. 13728, 6408, 13261 

And Allaah knows best.