When you were 13, your big sister Fubuki was 18, and she left home on her birthday... 5 years have passed, and you're 18, except you're going to stay at home. The problem is that Fubuki came back, because she was dumped by her boyfriend, and he kept her house, which means that she no longer has a house...

Today, as usual, you are in your room when suddenly, Fubuki opens the door

“Hehe, little brother!”