The husband of one’s sister’s daughter is not a mahram
Question

                With regard to my sister’s daughter’s husband, is it permissible for the to interact with him as I interact with my sister’s sons? What are the boundaries of interacting with him?.
Praise be to Allah.
      The husband of one’s sister’s daughter is not a mahram, 
      rather it is haraam for him to be married to a woman and her paternal or 
      maternal aunt at the same time, because of the report narrated by 
      al-Bukhaari (4821) and Muslim (37) from Abu Hurayrah (may Allah be pleased 
      with him) who said: The Messenger of Allah (blessings and peace of Allah 
      be upon him) said: “It is not permissible to be married to a woman and her 
      paternal or maternal aunt at the same time.” This does not mean that he 
      becomes a mahram to her paternal or maternal aunt, rather he remains a 
      stranger (non-mahram) to them. 

    The scholars of the Standing Committee for Issuing Fatwas 
    were asked: 

    I have a sister through my mother and father (full sister) 
    and one of her daughters has got married. Is her husband regarded as a 
    mahram to me or not? 

    They replied: 

    The husband of your sister’s daughter is not regarded as a 
    mahram to you, so it is not permissible for you to shake hands with him or 
    to be alone with him or to travel with him, unlike a mahram. End quote. 

    Fataawa al-Lajnah, 17/238 

    Based on that, it is not permissible for you to interact with 
    him as you interact with your sister’s sons, because your sister’s sons are 
    mahrams to you, but the husband of your sister’s daughter is not a mahram. 

    For more details on who is a woman’s mahram, please see the 
    answer to question no. 5538. 

    And Allah knows best.