The Princess recognized Gu Yunjiao as her sister because Prince Kang mentioned it to her last night.
The more Prince Kang interacts with Jiang Youzhi, the more he appreciates Jiang Youzhi's talents, wholeheartedly wanting to befriend him.
As a vassal king, Jiang Youzhi associating too closely with him is not necessarily a good thing.
Once Jiang Youzhi is marked with Prince Kang's imprint, even after passing the imperial examinations and entering the court, he may be feared by the Crown Prince's faction and find it hard to be appointed to important positions.
Prince Kang's influence in the court is very limited; maybe initially he can provide some help to Jiang Youzhi, but beyond that, it's uncertain.
Jiang Youzhi, being such an intelligent person, naturally understands this reasoning. Yet he chooses to associate with Prince Kang, making the Prince think even more highly of him.
