Ma-Ma-Go-To

The protagonist is an ordinary student. Next door to him lives a younger girl who is his childhood friend. She loved to play with him and call him "onii-chan". Now that they're a little older, she gets lots of attention in school. One day, her parents and the protagonist's parents had to go abroad together. For three weeks she and him will be alone. She says, "Until our parents get back, let's play house together and keep it a secret from everybody." Thus begins their exciting three weeks of virtual marriage. [translated from Getchu]