
First bc she's old, so I don't think it's possible for her to have baby of her own or her bloodline. Also for the reason, that was hinted in some chapters, that she was jealous of jinyao that bc he's first born he got everything. So now if she had the first child (according to her) of jinyao, and raised it. She or that kid (technically she bc child would be small, so she'll handle everything) so she can become the one who inherits everything.

It's simple, bc She doesn't want kid ,she wants the kid of jinyao's (who would be his first kid) who will have the power or authority to inherit all the property. So basically she wants inheritance and she can't get by herself (bc either way she'll get, that kid won't be first born of first born and their bloodline as well) so she has stolen the sperms thinking its jinyao's.
Even if she think that's could be fengs baby, why did she have to go that far...?!
Like stole the egg from there?!?!... If she actually want a baby she could have her own...?