Cousin marriage is proscribed and seen as incest for Hindus in north India. In fact it may even be unacceptable to marry within one's village or for two siblings to marry partners from the same village. The northern kinship model prevails in the states of Rajasthan, Gujarat, Uttar Pradesh, Odisha, West Bengal, Bihar, Jharkhand, Madhya Pradesh, Uttarakhand, Haryana, and Punjab. However in south India it is common for Hindu cross cousins to marry, with matrilateral cross-cousin (mother's brother's daughter) marriages being especially favored. The southern kinship model prevails in the states of Andhra Pradesh, Telangana, Karnataka, Kerala and Tamil Nadu.

Last time I broke the myth and told you that, for hindus from karnataka , beef is a delicacy. Now widen your knowledge in this area too, cross cousin marriages in south indian hindus a normal thing which is also the same thing practiced by many different Nepali cultures, only cross cousin though. Don't try to be master of genetics and spew here without doing some research though otherwise I have a bunch of scientific papers to quote here too. At least I assume that u believe in science and you must have studied in about Gregory Mondel in 9th grade biology class. you can easily understand it if u use that knowledge. No need to argue here.

Now Muslims case are extreme, they allow parallel cousin marriages too, which in long run, genetics defect case could be valid.

Let me push the story from revered Mahabharata ,

In Mahabharata Pandu’s son Sahadeva(Son of Madri, Pandu’s second wife) from Pandava clan had married his Maternal cousin Vijaya, the daughter of Dyutimat, the king of Madra. Which means as per Ancient Hinduism if your Maternal uncle has a daughter and your parents and your maternal cousin’s parents give their consent to marry her then you can get married, but if you’re a girl and your maternal cousin’s a boy then in that case he can’t marry you, which is strictly forbidden.