I thought that aunt was nice with jinyao? So she stole the sperm for power? This is gettin...

Yes she was pretending to be nice because she didn't have any means before (because she's infertile). Then she found out that Jinyao had performed artificial insemination, so she planned to steal that sperm and claim that baby. As explained in the current chapter, it didn't have to be her egg, she could adopt as long as the baby had their family's genes.

But Rourou was already born and went to Yiyi, then Yiyi's sperm has already been utilized so it already is an embryo. That embryo was stolen by aunt.