Swallow became Yongqi's sister.

Princess Zhu Huan, during the development of the plot, found that the swallow was the daughter of the emperor and she became Yongqi's sister, so she couldn't fall in love and get married, otherwise it would be incest.

Later, the plot was further foamed. Swallow is not the emperor's own daughter, but Wei Zi is. Therefore, the swallow and Yongqi are not related by blood, so there is no need to be sad. On the contrary, Wei Zi and Er Kang are real cousins.