He wants to marry the daughter of his sister's husband

Dear Brothers & Sisters,
As-Salaamu-Alaikum wa Rahmatullahi wa Barakatuh. (May Allah's Peace, Mercy and Blessings be upon all of you)
One of our brothers/sisters has asked this question:
Can I marry the daughter of my brother-in-law (my sister's husband) but not my sister's child ? 
(There may be some grammatical and spelling errors in the above statement. The forum does not change anything from questions, comments and statements received from our readers for circulation in confidentiality.)
Check below answers in case you are looking for other related questions:
Is the husband of her husband's daughter a mahram for her?
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Answer:

Praise be to Allaah.

It is permitted, because she is not one of the mahrams mentioned in the aayah (interpretation of the meaning):

“Forbidden to you (for marriage) are: your mothers, your daughters, your sisters, your father’s sisters, your mother’s sisters, your brother’s daughters, your sister’s daughters, your foster mothers who gave you suck, your foster milk suckling sisters…”

[al-Nisaa’ 4:23]

What is listed among these mahrams is “your sister’s daughters”, and the girl – in this case – is not the daughter of your sister. So there is no reason for this marriage to be haraam, either on the grounds of blood, breastfeeding (radaa’ah) or being related by marriage. And Allaah knows best.

Whatever written of Truth and benefit is only due to Allah's Assistance and Guidance, and whatever of error is of me. Allah Alone Knows Best and He is the Only Source of Strength.
Your brother and well wisher in Islam

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