Jessica - Siwon, Krystal, Jaehyun and Chenle are her siblings.

Jaejoong - Jiyong and Jennie are his younger siblings.

Siwon - Married man. Jessica, Krystal, Jaehyun and Chenle are his younger siblings.

Jiyong - Jaejoong and Jennie are his siblings.

Krystal - Siwon, Jessica, Jaehyun and Chenle are her siblings.

Jennie - Jaejoong and Jiyong are her older siblings.

Jaehyun - Siwon, Jessica, Krystal and Chenle are his siblings. 

Chenle - Siwon, Jessica, Krystal and Jaehyun are his older siblings.
His mother is actually Chinese. So the other 3 are only his half-siblings. *More details will be revealed in the story.

Minor Characters:

Jessica's bestfriend: Tiffany (daughter of Mr. Hwang, their grandpa's secretary)
Tiffany's girlfriend: Taeyeon

Tiffany's friends (soon to be Jessica's friends also): Sunny (daughter of Mr. Lee, their family doctor and Mrs. Lee a close friend of their family), Hyoyeon, Yuri, Sooyoung, Yoona and Seohyun

Krystal's friends: Kai, Victoria, Amber, Luna and Sulli

Jennie's friends: Jisoo, Rosé, and Lisa

Siwon's secretary: Jieun

Siwon's wife: Hyejin

The chairman grandfather: Choi Eun

The wife of the chairman: Lee Mija

Oldest daughter of the chairman, mother of Siwon, Jessica, Krystal and Jaehyun: Choi Inhye

Husband of the oldest daughter, father of Siwon, Jessica, Krystal, Jaehyun and Chenle: Jung Kiha

Second daughter of the chairman, mother of Jaejoong, Jiyong and Jennie: Choi Kyunghwa

Husband of the second daughter, father of Jaejoong, Jiyong and Jennie: Kim Dongil

Youngest daughter of the chairman, mother of Ella: Choi Siyeon

Husband of the youngest daughter, father of Ella: Ronald Gross

Youngest grandchild of the chairman: Ella

Not all of them are from their groups in real life. Only Chenle is, while the others have different occupations but still celebrity.