Ai no Kakezan

When Ringo and Lemon's mother died of cancer, Ringo felt she had to step up and be a substitute mother for her little sister. That's why she gave up her love for Tsubasa, who became Lemon's husband. Instead she is marrying Takao, with whom she thinks she will finally be happy. However, Tsubasa suddenly tells her about his true feelings...

(Source: natalie.mu, translated)