Natsume, who works at a game company, learns of his new step-sister the day of his mother's wedding. Just as he starts interacting with the game loving girl, he finds out the feelings the other siblings hold towards this girl…
2 Volumes (Complete)

It was really good in the beginning, and it's cool that it's from a guy's perspective. Then it starts to not make sense anymore.
He just randomly falls in love with her? At first he's like, oh yeah she's my sis that's cool, idk why all my bros are in love with her, and then suddenly HE tells her he's in love with her after some stupid predictable shoujo drama things happen.
Could have been good but it was really rushed. Maybe it will make sense if you read the original novel/game/whatever this was based off of.
It's refreshing to read a shoujo manga from a young - and male! - adult's perspective. The way he treats Ema like family, without any ulterior motive (so far anyway, I'm only at ch 3) like romantic interest, gives this peculiar realistic feeling to me. I guess I'm all for the protective big brother type! xD