Does she have to sit with her sister’s husband and talk to him?
Question

                Is the sister’s husband included in upholding the ties of kinship? Because I do not want to go to his house. I wear niqaab and he has seen me twice, accidentally on my part, but he did that deliberately. After that I cut off all ties with him. He is bothered by my wearing niqaab. Can I restrict my contact with my sister to phone calls only? She visits us all the time. Do I have to go and sit with him when he comes to our house, especially since he is the type of person with whom we have to sit and talk? Or can I say salaam only, even though I know that this will upset him? Or should I sit in the room by myself and not come out? I hope you can tell me what to do. Am I sinning if he sees me?.
Praise be to Allah.
    Your wearing the niqaab that covers the face 
    is something that is prescribed in Islam, rather it is obligatory, because 
    of a great deal of evidence that we have explained in the answer to question 
    no. 11774. Your sister’s 
    husband has no right to object to that, rather every believer should rejoice 
    at the spread of modesty and virtue. 

    There is no sin on you because of his seeing 
    you, because that was not intentional on your part. But he is sinning if he 
    looked at you deliberately, because of the report narrated by Muslim (2159) 
    from Jareer ibn ‘Abd-Allaah (may Allaah be pleased with him) who said: I 
    asked the Messenger of Allaah  (peace and blessings of Allaah be upon 
    him) about an accidental glance and he told me to avert my gaze. 

    Secondly: 

    Your sister’s husband is not one of the 
    relatives with whom you are commanded to uphold ties of kinship, neither is 
    he a mahram with whom it is permissible to shake hands or take off hijab. 
    Rather he is a stranger (non-mahram) to you, and you must cover in front of 
    him, and it is haraam to shake hands with him or be alone with him. Based on 
    that, you do not have to go to his house or get in touch with him, or sit 
    with him, but if you sit wearing niqaab and with your sister or a mahram of 
    yours present, there is no sin in that, so long as there is no cause for 
    doubt or fear of fitnah. 

    It says in Fataawa al-Lajnah al-Daa’imah 
    (17/420): 

    The sister’s husband is not one of a woman’s 
    mahrams, and he is regarded as a stranger to her. It is not permissible for 
    her to uncover her face in front of him, or shake hands with him, or be 
    alone with him, or travel with him. He is like any other non-mahram man. But 
    if she sits with him in the presence of one of her mahrams and whilst 
    observing hijab and covering herself, there is nothing wrong with that. End 
    quote. 

    See also question no.
    40618. 

    But our advice to you is not to sit with him 
    even if there is a mahram present, so long as he is deliberately trying to 
    see you without hijab and he wants you to sit with him and talk to him.  

    Thirdly: 

    This should not affect your relationship 
    with your sister, because she is one of the relatives with whom you are 
    obliged to uphold ties of kinship. If those ties can be upheld through phone 
    calls and her visits to you, that is sufficient. If she wants you to visit 
    her, then visit her to put her mind at rest, but choose times when her 
    husband will not be at home. 

    Fourthly: 

    From the above it is known that you do not 
    have to sit with your sister’s husband when he visits you, even if he wants 
    you to. But if you just greet him and leave, that is fine.  What matters is 
    that you adhere to the covering, modesty and chastity with which Allaah has 
    blessed you. 

    We ask Allaah to guide you. 

    And Allaah knows best.