Stealing the sperm is bad enough. By now you should know she is a two faced character.
Yes that's why I said that's the only evil part I know about her, at this point. But talk about being greedy or wanting her family's fortune for herself, that's still questionable. I'm just trying to say that it's hard to just her character as the ultimate evil here. If she mistreats the baby in the future then that's a different story. If we talk about stealing the sperm, technically Jinyao did it too in the beginning and have his own be swapped without Yi Yun's consent. I just want to give every character here the benefit of the doubt
Idk but this is just my opinion about Fengfeng's aunt. I can't generalize that she's actually up to no good.
Looking back, she's backing up Jinyao. She's against her brother and sister to the point she called them "clowns". She even has hopes for Jinyao to be the heir of the family. If we all reread again ch132, her main issue is the elders of the Feng family who value those who will be the heir the most. And it's because Fengfeng refuses to marry(thinking that he might have hurt by his "past relationship" lol but isn't actually the case) that he is at the disadvantage. So she thought she must step up so as to be a silent hindrance in her sister and brother's goal.
She found out Jinyao's secret about the sperm. She thought it was his. So, she took the action along her husband and stole it. Thinking that it'll be a direct descendant of the Feng's blood, this baby has the chance to be the heir.
For me, the only evil part i know about her at this point is that she stole the sperm and that's it. I can't actually say she's being greedy here and wants the fortune of her family all to herself as of yet. I'm waiting more what she has to do in the future.