When they first found out that Su Yan had fallen in love with a girl from the village, they had spent a lot of effort to separate them, and even said a lot of terrible things to her like how she was unworthy of Su Yan and that she had to know her place.
Then, for some reason, Ning Zhiyuan decided to make Ning Yaohua and Zhuang Lingyu retain Ning Xi's identity as their foster daughter. Although she was already part of the Ning family, her identity was not worthy to be associated with the Su family.
After they realized Su Yan was in love with the real daughter of the Ning family, Ning Xueluo, they were overjoyed, thinking that she was the real golden phoenix.
In the end, the roles were all reversed!
Ning Xi was the real daughter, and Ning Xueluo was the wild chicken trying to act as the phoenix. They had been deceived for five years. How else would they feel?