Subhadra was the sister of Krishna, and Arjun was the cousin brother of Krishna, As far as I know, Kunti was Krishna's Bua (father's sister).
So if they were blood relatives, How could Arjun and Subhadra marry?
Hinduism Stack Exchange is a question and answer site for followers of the Hindu religion and those interested in learning more about Hinduism. It only takes a minute to sign up.Sign up to join this community
Daughter suitable for marriage.
Manusmriti 3:5. A woman who is not of the six generations of the mother and of the father's gotra, is recommended to twice-born men for wedlock and conjugal union.
Kunti was biological daughter of Surasena but she was adopted by Kumtibhoja, the paternal cousin of Surasena.
Vaisampayana continued, 'There was amongst the Yadavas a chief named Sura. He was the father of Vasudeva. And he had a daughter called Pritha, who was unrivalled for beauty on earth. And, O thou of Bharata's race, Sura, always truthful in speech, gave from friendship this his firstborn daughter unto his childless cousin and friend, the illustrious Kuntibhoja--the son of his paternal aunt--pursuant to a former promise.
So, Kunti was not full sister of Vasudeva. The generation gap is is very much between the Subhadra and Kunti so their marriage is justified.