It is not permissible for a girl to marry her father’s maternal uncle, because her father’s maternal uncle is also her maternal uncle, so he is a mahram. 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”

[al-Nisa’ 4:23]

The scholars (may Allaah have mercy on them) have stated that the paternal uncle of a father is also the paternal uncle of his son, and the maternal uncle of a father is also the maternal uncle of his son. 

And Allaah knows best, 

See al-Muqni’ wa’l-Insaaf wa’l-Sharh al-Kabeer (ed. By al-Turki), 20/277.