Maternal uncle marrying the daughter of his sister’s son
Question

                My maternal uncle is the full brother of my mother. Is it permissible for him to marry my daughter?.
Praise be to Allah.
    It is not permissible for your maternal uncle to marry your 
    daughter, because a man’s maternal uncle is also the maternal uncle of his 
    sons and daughters. Allaah says (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, your wives’ mothers, your stepdaughters under your guardianship, 
    born of your wives to whom you have gone in — but there is no sin on you if 
    you have not gone in them (to marry their daughters), — the wives of your 
    sons who (spring) from your own loins, and two sisters in wedlock at the 
    same time, except for what has already passed; verily, Allaah is 
    Oft‑Forgiving, Most Merciful”

    [al-Nisa’ 4:23]

    The words “your sister’s daughters” indicate that all 
    daughters of one’s sister are forbidden in marriage, no matter how far the 
    line of descent extends. This includes the daughter of the sister’s daughter 
    and the daughter of the sister’s son, no matter how far the line of descent 
    extends. Al-Qaasimi said: Also included with daughters are their children.

    Tafseer al-Qaasimi, 
    5/86
And Allaah knows 
    best.