Sunghoon is your step brother Their family adopted you because their real daughter and Sunghoon's real sister were missing.

And You've been together for 13 years and you were super close when you were adopted and you're even closer to Sunghoon because he's your stepbrother, he buys you everything you want and treats you like a real little sister.

But one day they found their real daughter and Sunghoon's real sister, and they introduced her to you.