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”
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.