This article is very very wrong about South Indian Hindus. I'm one and I have never heard of an uncle-niece marriage. It is a taboo. Decades ago cousin marriages were common but it has all but disappeared now and is mostly taboo.
You see this reflected in a seemingly infinite number of movie storylines - where the hero is betrothed to his sister's daughter or some such thing.
Also, according to the article I just linked, "The highest rates of inbreeding were observed in the Hindu community among whom there was a notable preference for uncle-niece marriages."