Saw this one a long time ago but was recently reminded of it, and refound the source – those panels are pure genius. The whole story is below, and is quite steamy.
Manga is by 青木幹治 / Kanji Aoki, published in 漫画ばんがいち 2006年 11月号 / Manga Bangaichi 2006/11. The title is “お姉ちゃんの手を取って”, “Take Oneechan’s Hand”. He is a regular contributor to Bangaichi, but seems yet to have released a compilation.