from ebookjapan:

High school student Hoshino Ryou lives together with his father. Unfortunately, the family business, a bath house named Hoshinoyu, is on the verge of going under, because his father is both lazy and a pervert, often just sitting around peeping at the female customers. As Ryou tries desperately each day to keep Hoshinoyu afloat, three visitors from outer space suddenly show up. They are actually members of the royal family on a planet far from Earth, but they were forced to flee after a coup d'etat.

Since they had nowhere else to go, Ryou reluctantly allows Prince Kanata, his older sister Princess Yuuri, and their attendant Jii, to live with him at Hoshinoyu. On the positive side, they possess advanced alien technology: small devices, about the size of coins, capable of releasing enormous amounts of energy incredibly quickly. On the negative side, they are pampered royalty who grew up in a low-gravity environment, so weak that 3 minutes of sweeping the floor has them bedridden. An unbelievable new life is about to begin for all of them!