The End-of-Year Lazy Warrior

Summary
While their mother is on a trip overseas and Rumi is going to a Cram School, Bossun is left on his own for a few days of the winter holidays. Adamant that he can look after himself he is determined to enjoy his freedom doing whatever he wants. However, he eventually gets bored and starts playing video games and tries a new game that his mother had been hiding (as she found it too addictive) - Monster Fantasy

Three days later, Rumi returns home and finds her brother in a semi-comatose state and addicted the game after playing for 60+ hours straight. Unable to rouse him she calls the strictest person she can think of - Tsubaki. He in turn is equally unable to rouse his brother and starts to watch Bossun play. Having never played a video game before he becomes fascinated and after being told by Bossun that it has Co-Op play, he becomes equally hooked...

Together in the game the brothers actually work together and bond (without realising it) but midway through are defeated by a mysterious character, awoken from their stupor (after playing for a whole week nonstop) they find that Bossun's mother Akane had joined the game to defeat them as she saw it as the only way to rouse them.