Double Wish(2004)

W: Wish

Based on a game by Princess Soft

The main character, Junna has a twin sister Senna. He is an ordinal student of an elite school. However, in the past, a traffic accident deprived him of his parents and his memory. Junna survived the accident, and he has lived only with his sister though he has been looked after by his relatives.

And present...
The life with Senna in the same high school is so pleasant that he can forget the severe past. Because he has been in the world where there is only Senna, his lives in this town, such as the beginning of a new life, new environments, and the meetings, are so refreshing.

However, he begins to recall the memories he lost in the accident. Though he enjoys happy and pleasant days, he is tossed by the past, the present, and the future. What is the truth hidden in his memory?

(Source: AniDB)