If you're looking for manga similar to Ashita no Familia, you might like these titles. All recommendations are made by Anime-Planet users like you!
Hiro Hiyorimi is a boy who moved to the city to live with his sister; but as he was walking around town one day, he sacrificed himself to protect a girl from being hit by falling beams, and subsequently lost his life. Her name is Hime, and she is a princess who has the power to revive the dead with the flames of life. In return for reviving Hiro, he must work as a servant to protect her from any harm that comes her way. Yet even though Hiro now has a partially immortal life, his flames of life must be recharged daily or he will die yet again. Hime’s life is targeted by her siblings because the throne of the royalty is at stake, so Hiro must step up as a faithful servant to protect her life at all cost!
In both series a human boy lives in this big mansion with a number of monster girls for some reason. Oh, did I mention there are the monster girls in both series?
Anyway, Kaibutsu Oujo has cooler characters and (probably) better action scenes, but Ashita no Familia doesn't come all that far behind when it comes to being entertaining.