Si Ning withdrew his hand from Xiu Zhicheng's hold due to the shock and weight of Xiu Zhicheng's statement. He was trying to have power equal to the Emperor and Xiu Zhicheng announcing to make him the Empress was just the answer to it all but if Xiu Zhicheng takes half of his army to join the Imperial family army, even after marrying the Emperor and becoming part of the Imperial family doesn't mean he was really part of the Xiu family and he would have no control over the Imperial army which means he would become crippled. It was just as Xiu TianZhao said, the Imperial family were afraid of him having more power than them and the Ministers were also afraid of him having more power than them in the court.
Seeing Si Ning's reaction made Xiu Zhicheng took the hand he withdrew and held it to his chest, rubbing it slowly to gain Si Ning's attention before using his thumb to gently rubbed Si Ning's eyebrows in a way to make Si Ning stop frowning.