I don't know clearly when this rumor came up. But, I had been searching that the rumor began to come up at about October 29th. I know this rumor from twitter and I have feeling that the rumor came up from twitter.
As you see, the rumor slowly began to come up after the bottom of the tweets. I don't know if he/she got the rumor from where.
And the rumor has become increasingly widespread in every tweets even in real life.
But, first... you should know what is this girl's name.
I've searching about her and found some Korean asked the same question on Naver KiN (Similar to Yahoo! Answers). Here is...
The questioner asked, what is the name of the girl who featured on "War of Hormone" MV?
And the answerer answered, she's a rookie model, her name is Jeon Yujin. And also the answerer gives the link of her facebook.
So, the people have the thought that she's Jungkook's cousin because they have the same surname... am I right?
Before I found some Korean's question, I had found her facebook first. At first, I was doubtful, was it really her own facebook or faker facebook?
Previously, I wanted to know about her by searching on three top search engines in Korea (Daum, Nate, and Naver). But the result were disappointing. There was no news about her except Daum - although a bit news about her on Daum too. Maybe it because she's still rookie for now, so too hard to find out the news about her.
Then I've continuously searched, and I think it's really hers.
That's her profile facebook. She also made her fanpage facebook so that her fans can get touch with her. Here is...
And then, she said on her profile facebook as well as her fanpage that she has made Instagram account.
On her instagram, there was one that curious and wanted to know is she really Jungkook's cousin or not.
On her other photo, many ARMYs (BTS fanclub name) asked the same question "Are you really Jeon Jungkook's cousin?" to her.
And some even asked it in Korean language.
"You aren't Jungkook's cousin right? kkk"
"I'm not (her) cousin."
Also, there's no photo Jeon Yujin with Jeon Jungkook at all. Even if they're really siblings, why there's no photo after "War of Hormone"s MV at all, right?
So, the conclusion is Jeon Yujin isn't Jungkook's cousin. It's just a rumor.
And, in Korea, there's no one talked about this rumor on Twitter even on every Korean search engines. So, this rumor made up by International fans. Maybe, because they think they have same surname, so it could be his relation, but because he just have one older brother so they think it could be his cousin. LOL.
Alright, I also found that not a few people curious about how old she is. Yeah, there's no full profile about her on three top search engines (Daum, Nate, and Naver) even there's no news about her how can I know her full profile? LOL.
But, from I see on her profile facebook, her hometown is Yesan - a county in South Chungcheong Province, South Korea.
And, I knew her birthdate. Well, even if I don't know what's her birthyear but at least you can give her a happy birthday. Her birthdate is July 11th.
And here is the proof, many her friends give her a happy birthday and also she has ever posted a photo with birthday cake.
Okay, enough LOL.
The conclusion is it's just a rumor and she has confirmed it on her own Instagram.
So, please everyone, I hope you all didn't trapped in the rumor's circle. Just don't be too trusting the rumors, and don't spread more the rumors without the basis.
This source by my analysis, and thanks to Google for the map LOL.
If you want to repost this, please give me a credit.