onee-chan_no_te_wo_totte
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 boobs (post #70972).