Ashita mo Kitto Koishiteru

Taken from Michex:

MITSUKI is on top of the world when IMAI, her former tutor and sister's friend, agrees to begin dating her. Even with the seven year age gap, she's determined to make it work, even though she hardly ever gets to see him. But her happiness slowly gives way to doubts when a year passes without ever breaking down the barrier she feels between them. Does Imai-san even like her at all? Or is the person he's really in love with her older sister and he's just going out with her because he didn't have the heart to turn her down? Maybe he doesn't take her seriously because she's still in high school? When she's with him, he always seems more of the serious type---but what is this about his playboy past?!