He is not their own son.
He was just three years old when his parents and his elder brother died in a car accident. His father's side of family was very popular and rich. His father's elder brother took over the company of his grandfather as Jungkook's father started his own business as the 'Jeon Electronics' which gradually became one of the best companies in Korea.
His father and uncle had a very good relationship and he was loved very much.
Unfortunately when he was three years old, his parents went for a business meeting, having him at his grandparents house. They both died in an accident while coming back.
Since his uncle loved him very much, he decided to let him live with them and love him as their own son. But not everyone in his house was fond of the idea.
His aunt did not like someone else to have a share of the love and property which was only meant for her son.
But property was not a big problem for him because , his father had invested enough money for him before dying which could last him a lifetime.
His grandparents still decided to give him half of their property which was, of course , not like by his aunt.
That is why she didn't treat him well. His 'brother' or cousin Jungwoo, had always been a spoiled child so he didn't care about anything. He never treated Jungkook badly, well except the bro fights and all, where sometimes he will tell him how grateful he should be that he shared his parents with him or else he would be known as an orphan. Well it hurt Jungkook very much.
He was loved by his uncle, but he couldn't have the love of a mother because his real parents left him when he was too young. he tries to make his aunt like him in any way possible but she never even looked at him properly. She was not among the one's who would scold and yell but she was very cold towards him.
His 'parents' now lived in America with his grandparents while he and his 'brother' were sent to Seoul to be able to graduate and have a good job.( they were rich but still wanted to have their own carriers)
He was a bright child since childhood. When his 'parents' left for States , he was in 9th grade, he and his brother started to have more freedom. He started hooking up since tenth grade, but still maintained his academics. While his brother's grade decreased gradually.
He hooks up but not as often as his brother. Sometimes he gets fed up of his brother's habits but still stays because he didn't want to dissapoint his uncle.
But everything has a limit, don't they?