The boy was 10; turning 11 when he first heard his biological mother was pregnant. A few months afterwards, a few months before his birthday, he was snuck out the mansion, against his own will really, to meet his mother and new sister. She was a tiny, sickly little thing, born blue from the cold, the doctors said, despite the warmth of the hospital (the Ninth had been kind enough to supply her with such facilities) and the climate outside.
Before Xanxus left again he called her 'Ciel'. Her eyes were a fierce bright orange like the sunset outside.
Though Xanxus would not meet her again for quite a while, it would not be a lie to say he and Ciel spent all their lives together (or as much of their lives as possible what with Xanxus getting frozen some years later and all).
It might be said that a year later, Xanxus' mother's birthday present to him was Ciel for on the morning of the tenth of October, there was a buzz about the mansion. A box had been left at the front door of the Vongola mansion and it had been addressed to the Ninth. It contained a small child. Xanxus recognised her as his sister at once (practically putting everyone present into shock, since when did Xanxus care about such things?)
And that is the beginning of how Ciel became a resident of the Vongola mansion and in turn, the Varia's resident little sister and mascot.
(The last part was lie. Everyone knows the Varia's mascot is Squalo.)