It's weird to come across a shojo manga that feels so much like a shonen ecchi manga without actually being an shonen ecchi manga.
CHERRY JUICE (Cheri Jusu), by Haruka Fukushima. First published in 2004 and first published in North America in 2007.
PLOT:
Minami and Otome are seemingly as normal as step-siblings can be, even if Otome's best friends all envy her for having such a hot, popular step-brother. Then Minami's best friend, Amane, confesses to Otome and kisses her. Now he's not sure just what the nature of his feelings towards his step-sister are, and Otome is starting to feel equally confused.