Futsuu no Koiko-chan

From ShoujoHearts: Koiko has lived her life under an overly-attached mother and a distant, neglectful sister. While she dislikes the idea of drowning in it, Koiko still wants to be in love, so she settles for being an average high schooler with an average boyfriend and an average relationship. One day, the most popular boy in school, Tsurugi, sends her a message: "Your boyfriend is cheating on you." This is where our story begins!