Based on the erotic game by Princess Sugar. 
 
Due to his father’s work, Yahiro began living together with his imouto Io. Walking home after wrapping up the enrollment procedure at their new school, they noticed a large truck stopped outside of their house. Since they had already moved everything in, they wondered what it was doing there. They found the door unlocked and nervously went upstairs worried that there might be a burglar. Instead they found a girl pushing a giant piano with all her might… while in her underwear. They stood dumbfounded at the absurd sight. Why was there a half-naked girl in their house? Soon she noticed them and yelled at them to get out. A few minutes later, she came out in a beautiful dress. 
 
“I am deeply sorry for my earlier actions. My name is Ruria Fondina. I have come here to live with you as family.” 
 
She was a princess from a certain island nation. That was only the beginning as many other princesses showed up at their door one by one!