Prepare For Trouble, Make it Double

WARNING: Genderbend

Jeongyeon and Nayeon got married and had fraternal twins, Chaeyoung and Joohee (Tzuyu). They had an argument, she left to go back to Korea and took Joohee with her. Jeongyeon kept Chaeyoung with him, raising her with the help of a nanny. But 12 years later at an important event in New York, Chaeyoung and Joohee met, realizing they were sisters. They became best friends with only one thing on their mind: getting their parents back together.

Yes, this is based off The Parent Trap.