Praise be to Allaah.
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
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?
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.