nah i don't remember a scene where they say the ovum used is of Fengfeng's aunt because technically, the aunt assumed that the sperm is Fengfeng's so irregardless of who donated the ovum, the child born would have the Feng family genes. Unfortunately for the aunt, she and her minions didn't count on the sperm to be someone else's because as I remember Fengfeng already made the switch which was how Rourou came to be.
yeah i don't think she'll risk it, assuming the sperm is Fengfeng's, the baby would be like the result of an intermarriage between family members. besides she doesn't know Rourou exists so she probably thinks her plans are foolproof. i'm just patiently waitong for when they dna test the kid, she's gonna fall harder than conniving siblings.

Guys I just see the Raw and YiYi baby is coming next chapter