Is it permissible for her sister’s husband to stay overnight with them in their house?
Question

                My sister and her husband stay overnight with us. I am still unmarried and am older than her. Our flat has two bedrooms and a living room.

 Is it haraam for them to stay overnight with us? Because I am not yet married and when he is here he uses every part of the flat. Is it haraam to go out shopping with him to buy things for the house?.
Praise be to Allah.
      Your sister’s husband is regarded as a stranger 
      (non-mahram) to you, so it is not permissible for you to be alone together 
      or for you to sit with him without a mahram being present, or to go out of 
      the house with him, or to uncover your face in front of him. He is like 
      any other non-mahram man. 

    It says in Fataawa al-Lajnah al-Daa’imah (17/420): 
The sister’s husband is 
    not a woman’s mahram, and he is regarded as a stranger (non-mahram) to her. 
    It is not permissible for her to uncover her face in front of him, or to 
    shake hands with him, or to be alone with him, or to travel with him. He is 
    like all other non-mahram men. But if she sits with him in the presence of 
    one of her mahrams and observing proper hijab, there is nothing wrong with 
    that. End quote. 

    The following question was mentioned in Fataawa al-Lajnah 
    (17/160): 

    My brother’s daughter and I are married to two brothers, and 
    I am his cousin. We have lived together in one house for a long time, and I 
    sit with my husband’s brother wearing hijab that is not complete, in the 
    sense that my face is uncovered, but I do not make a wanton display of 
    myself (tabarruj). What is the ruling on this? 

    The committee replied: 

    A woman is obliged to cover her face in front of the husband 
    of her female cousin and the husband of her sister, and the husband of her 
    brother’s daughter, because they are strangers (non-mahrams) to her. It is 
    not permissible for her to uncover her face in front of them, as is the case 
    with all other non-mahram men, because the face is one of the most important 
    ‘awrahs that must be covered in front of men, because it is the site of 
    fitnah (temptation) and looking. End quote. 

    His simply staying over at your house is not haraam in and of 
    itself, if all precautions are taken and there is no danger of anything 
    happening that Allaah has forbidden. 

    But, since the house in which you live is small as you 
    describe, then your sister and her husband should look for another house and 
    move there, if they are living with you on a permanent basis, or reduce the 
    length of their visits to you, if they are just visiting you. During those 
    times you must not be negligent with regard to observing hijab in front of 
    him, even if that is difficult for you, because this is the command of 
    Allaah and He knows best what is fit for His slaves. 

    And Allaah knows best. 

    See also questions no. 
    6408, 13261,
    13728,
    40618.