Someone please explain to me did old lady steal the embryo? WHY???

She thought it was Feng Jinyao's, and wanted to steal his kid.

Because she thinks it's Jinyao's baby. Since Jinyao secretly switched sperm donors to let Yiyi raise Rourou, she stole the embro believing she was stealing her nephew's son.

This way, her "son" would become the next heir to the family business.