Gochuumon wa Usagi desu ka?? Dear My Sister

Cocoa visits her family, in the mountains, after quite some time again after her sister Mocha wrote her, through a letter, to come back soon. Her friends will have to spend a week in the town without her, while Cocoa has time with her family that she hasn't seen for a long time. But Chino, Cocoa's self proclaimed little sister, wants to go watch, with Cocoa and all her friends, the fireworks in the fireworks festival, which is coming up in less than a week.