Naisho no Koibito(1998)

Naisho no Koibito

This is another typical story about a pair of adopted siblings who fall in love with one another. Jun, who was adopted into Hinako's family 4 years earlier after the death of his parents, never looked upon her as an 'older sister.' Jun kept his growing feelings for her hidden. But what are Hinako's feelings for Jun...?

(Source: Wataame Shoujo)