Because along the way, Xia Hong frequently took them to the Slave Village for tours, evidently to make them feel the difficulties and hardships of the lower classes. Coupled with occasional inquiries into their views on the Six-class System, the intent to guide them was too obvious. Not only Su Xing'er but even her three brothers, Su Jing, could realize it.
Of course, even if they figured it out, they showed no resistance. Aside from Xia Hong's immense favor upon them, they also deeply detested the Six-class System since their parents died because of their status.
Naturally, these thoughts could only be buried deep within for the time being.
As their master said, overthrowing a system that has been running for hundreds of years and has long been ingrained in people's minds is not something that can be done overnight. It's extremely difficult for him, let alone for the four siblings.
