Kaguya is Chitose's older sister and Tokiko's granddaughter. That's why they have a moon stone at the mansion in the First chapters. Chitose says the moon stone is something special do her nee-sama because it conforts her. The only girl who really cares about going to the moon is Kaguya. Also, Kaguya is Very small and knows all the basics of programing, Tokiko was the minister of education and know how to program in C. I think Chitose has two older sisters, a blood one (Kaguya) and the one who she deply admires (Tsukasa). At last, but not least, Chitose and Kaguya look alike, their hair is kinda similar.