Thunder ( MBLAQ) and Dara( 2NE1)
Seungri (big bang)  and hana (her group has not debuted yet)
NS yoon G(solo singer) and ji-young (Kara)  (they are cousions)
Hyoyoung (5-dolls) (Older) hwa-young (younger) (former T-ara member still active in k-pop actress) (twins)
Juhee and sunghee (twins from wink)
Youngmin (older) and Kwangmin (younger) (twins from boyfriend)
Kibum (former u-kiss member still active in k-pop) and hyungjun (former SS501 member now solo)
Zico (block B) and Taewoon (co-ed school)
Boram (t-ara) and ram ( D-unit)
Jessica (snsd) and Krystal (Fx)

Leeteuk (super junior) and In-young (actress)

junsu (jyj) and juno (solo singer)

SoRyong and DaeRyong  (twins make up the duo tasty)

Mir (MBLAQ) and go eunah (actress)

Taeyang (big bang) and Hyun Bae (actor)

yoochun (jyj) and yoohwan (actor)

ilhoon (BTOB) and  Joo who (solo singer) (brother and sister)

Kwon Young Don andKwon Young Deuk (YG backup dancers meant to be in YG's new boy group)

jae jin (FT Island) and Chaewon (actress)
Bom (2ne1) and Go Eun (famous celloist)

nichkhun (2pm) and Nichchan (actor)

Top (big bang) and Hye Yoon (i think she is a model)

sungyeol (infinite) and Daeyeol (woolim back up dancer)

woohyun (infinite) and boohyun (famous chef)

 

these are all the people in k-pop who are related that i know of if their are any others let me know

 

28 total pairs of siblings in the korean entertainment industry