Disclaimer: I do not own La Corda D'Oro. Or anything else related to this anime... Happy now??? Jeez..... :P
When Shinobu was 10, Kaho was 3 then, Kaho's parents took Kaho, Kazue, and Satoru to a public school are, rather than a private school, richy-rich area, separating the siblings to see if they would remember each other after two years of separation.
(In present time, Shinobu and Satoru are 24, Kazue is 22, Kaho is 17, and Hitomi is 13.)
They gave Shinobu and Hitomi the maiden name from their grandmother on their mom's side, and Kaho, Kazue, and Satoru got the maiden name of their father's mom.
But, when their parents realised how happy Kaho, Kazue, and Satoru were at their new school,s it pained them to have to transfer them back to their old schools where they had been teased and bullied.
Kaho's parents would often leave on long "business trips" to go visit Shinobu and Hitomi. Kaho's parents would leave for months at a time, leaving Kaho, Kazue, and Satoru under the care of a nanny.
The same would happen to Shinobu.
Once Kaho's mother became pregnant with Hitomi, Kaho's parents stayed with Kaho, Kazue, and Satoru until Kaho's Mom's bump was visible.
At that time, Kaho's parents both went on a "business trip" and went over to Shinobu until the baby had been born, leaving Shinobu after the baby was a month old.
When Kaho was in Middle School her parents got in a car accident, and Shinobu heard of it, thinking that all of his other siblings had died in the crash.
Of course, Hitomi had never heard of her other siblings, and therefore was never worried.
(A/N: And that concludes my explanation chapter... Ja ne!!!! ~Franfran)