Haru Yo, Koi by Kaori Saki
(Can't find a nice version of the Japanese cover.)
Summary from Baka-Updates, from Manga Jouhou:
Takashi Sone is living on his own while he is attending college. Out of the blue, his parents tell him that his sister, Mafuyu, is moving in to live with him and that she will be attending a high school in his area.
When she arrives, he finds out that she was kicked out of her old school for getting caught making out with another girl. Soon afterwards, the other girl arrives, having thrown away everything she owns in order to run away and live with Mafuyu.
Anyone know where I can find this manga DDL? BT'ing is too slow @_@;
Scanlations are available for online reading on [replacer_a], though it is incomplete. And you have to be above 18. ~ImotoChan