It's about a boy and a girl who move into the same house and live together like step-siblings.
In the end, they fall in love with each other and get together.
I think the girl was very good at school and stuff, like a perfect person. During the manga, she also got another boyfriend and the brother was jealous.
The genre was romance, of course, I think there is also some ecchi and shoujo in it (can't remember properly).
In the very end, when they come together, they also talk that the boy has gotten taller and then they kiss.
Please, I need your help finding that manga! (there is no anime based on this manga, afaik.)