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: anilist.co)
~~ Adapted from the manga series "Ai no Kakezan" (愛の掛け惨) by Osamu Suzuki (鈴木おさむ).