Does Kim Hyun Joong have a sister? No.

We do not know if his mother would have liked  a girl, but she had only two (very handsome) boys.

So why is Kim Hyun Joong talking about a sister when he writes:

(..)
I texted my sister Yuna, whom I had only met once.
Sister Yuna helped me with my ridiculous project.

(..)

 

Because the expression “my sister” is here used as the expression “my brother”

 

Because any female presence in Kim Hyun Joong’s circle is often the subject of speculation, and placing this person where it should be, avoided curious or even derogatory comments about her.

It was his way of “protecting” Yuna.