Yara Greyjoy (Asha Greyjoy in the books) and her little brother Theon
From the television series, Game of Thrones, adapted from George R. R. Martin’s A Song of Ice and Fire novels
Cafe Endou is run by Tou Endou and his younger twin sister, Saya.
From the anime series Dagashi Kashi
Shuusuke Takanashi is a typical perverted teenager who collects porno magazines, stares at girls’ underwear, and plays eroge. His adopted sister, Nao, takes advantage of this, actively teasing him in the hopes that she’ll become the focus of his attention.
From the anime series Oniichan no Koto Nanka Zenzen Suki Janain Dakara ne!! (I Don’t Like You At All, Big Brother!!)
Asks the girl, Shiki, one thing:
Are you my sister?
From the anime series Arve Rezzle: Kikaijikake no Yousei-tachi (Arve Rezzle: Mechanized Fairies)