I Don't Like You(1999)

Suki ja nai kedo Aishiteru

Kouki Hanashima and Asahi Hidaka share a class-recognized love, although it's a relationship that seems to have its ups and downs. One day, out of nowhere, Kouki's older sister pushes her young infant son on the couple, saying that her duties as a stewardess requires her to be away for a month. Not only is the child named Asahi as well, but it seems that he's becoming a rival for the older Asahi! [From Tsuyu Translations]