Bucong Shuye was not closer to Zhao Shuling because she was not his biological mother. His biological mother had died when he was 3 years old. One year later, his father brought Zhao Shuling and married her. So, she had been his mother for the last 20 years.
At first, after Zhao Shuling gave birth to Bucong Jian and Bucong Nanyi, she tried to uproot Bucong Shuye's position as the heir to Bucong Family. However, she was not successful. Bucong Shuye was stronger than her sons. Bucong Guang was someone who prioritized strength. If Bucong Jian and Bucong Nanyi were as strong as Bucong Shuye, they might be able to take Bucong Shuye's position.
It was not like Zhao Shuling never schemed against Bucong Shuye but she never succeeded. The last thing, she schemed against Bucong Shuye, her Zhao Family almost lost their lives. After that incident, the Zhao Family never helped Zhao Shuling's scheme against Bucong Shuye anymore. Zhao Shuling quieted down after giving birth to Bucong Jiye.
