Can a boy marry to his aunt's sister's daughter(father's brother's wife's niece)

Niece of paternal uncle's wife. Paternal uncle and his wife and children are your sapindas. Paternal uncle's wife herself is related to you only by virtue of her marriage to your uncle. But her blood relations would not  become your blood relations unless your paternal uncle himself has married a blood relation. Hence the marriage is not prohibited. In Vetala Panchavimsati every story ends with a query and Vikramaditya gives correct answer to 24 queries. The 25th story ends in a query like yours and Vikramaditya was not able to give the correct answer.