Imouto de Ikou!

Imouto de Ikou! is a 2002 game made by 0verflow that later received an OVA adaptation.

Plot
Hisashi Yoshizumi lives at home with his father, having lost his mother at a very young age. One day, upon coming home from school, Hisashi learns that not only has an alien spaceship crash-landed onto his house, but that he now has a new mother and sister!

Main characters

 * Hisashi Yoshizumi (protagonist)
 * Mayuka Yoshizumi
 * Haruru Kurahashi
 * Fuyuno Yoshizumi
 * Akiho Tanemura
 * Seika Yoshizumi
 * Ikuko Yoshihara
 * Ena
 * Yuma
 * Kazumi Mizasawa

Secondary Characters

 * Takashi Yoshizumi
 * Jun'ichi Shiina
 * Kengo Ichijou
 * Riko Wanigaki
 * Nina

Makes an Appearance

 * Ai Ogata (from PureMail)

Trivia

 * The school in which the game is set, Sakakino Seiryou, is also mentioned in Summer Radish Vacation.