Yui's Neko Sister

Kimiko Hisakawa, the adopted younger sister of Yui Komori.
One day, her so called 'father', Seiji, had called Yui and Kimiko to his office. He told them that they'd have to live with some distant relatives from now on while he leaves to another country for some 'business'.

What will happen?
What kind of people are these 'relatives'?