So, Watanuki is Syaoran's 'replacement'? That would make him the 'son' of CCS Syaoran and Sakura, and Watanuki's parents are both supposedly dead already.
Same Side, Same Story
CC Sakura's magic square floats underneath Shaoran's feet. It is possible to rewind time but the offense is far too heavy. By rewinding time the princess may go down a different future. That also means that at the same time, because of that, the futures of all the people connected to the princess will change. Particulary, the future of those who knew about the princess' curse will change. And Shaoran must bear the greatest sin. Even with the rewind, he will not be by her side, will have his freedom taken away, his life exposed to danger and will be able to do nothing but watch. Even so, Shaoran chooses "one more time". (T/N: Meaning he chooses the rewind.) Soon Shaoran is rejuvenated to the way he was when he met the princess. And a boy appears in front of his eyes. He is the existence that was born when Shaoran rewound time, in order to fill up Shaoran's former position, the relationships he gave as payment. He is CC Sakura and Shaoran's son, not a brother but Shaoran himself. Fei Wang says that the existence of Watanuki, who was someone not originally supposed to exist, such distortion will bring pain to the parents who helped Shaoran and to Watanuki.