Miyuki

(Summary from here):

Wakamatsu Masato lives with his step-sister, Miyuki, whom he does not have blood relationship with. Their father is always working oversea (in fact, he has not appeared in the manga at all...) and left the two of them living by themselves in Toukyou. Miyuki is cute, nice, sensitive and very popular among guys around Masato. Masato has a girlfriend at school whose name is Kajima Miyuki, who is pretty and gentle, the ideal dream of every high school boy. It took years for Masato and his sister to realize whom they love and care most after a lot of events. The manga is very nicely written, with excellent layout, interesting story and character developments, refreshing art style and filled with youth and emotion. It also shows the dramatic improvement of the author's skill.