Also known as: お姉ちゃんの手を取って
A short Eromanga by Aoki Kanji. Later it was published as a part of his adult manga book, "Sayonara, Oppai" in 2009. Note that this is not an Incest manga (they are childhood friends).
Best known for a frequently parodied scene known as Knee Boobs, in which Yumi presses her knees to her chest to pretend she has Breasts (post #70972).