Sonan Jyanaiyo

From Anime Waves:

Meet Shizuki Maku - to say she looks "above average" would be a slight overstatement, but it's especially bad compared to her older twin brothers, the high-school heartthrobs Retsu and Tetsu. She always wondered whether or not she really belonged in her family, but things take a shocking twist when her brothers plant the suggestion that they may not be related after all.

Now she's supposed to pick one of them to be her boyfriend, all the while defending herself from all the other girls at school who want to keep her "brothers" for themselves! Life isn't easy when you're the ugly duckling!

Sonnan Ja Nee Yo received the Shogakukan Manga Award for shoujo manga in 2006.