What is the ruling on a maternal uncle marrying the sister of his paternal niece's son?

If there is no prohibited degree of kinship by suckling or otherwise between Khalid's maternal uncle and Khalid's paternal sister, then the marriage is valid, because there is no prohibited degree of kinship by lineage or affinity. The fact that Khalid's father is married to the man's sister does not prohibit the man's daughters from marrying Khalid's sister.