Actually it showed before her own intentions in a previous chapter. It’s true that she does really want a child of her own since she’s been trying with her husband for so long- which I feel for her. However she did deliberately steal the embryo. She doesn’t know it’s yi yun’s and believed it’s feng’s embryo. Regardless she knows she took someone else’s who wasn’t aware of it, which she wasn’t supposed to do.

I honestly don't think Feng's aunt had *deliberately* stolen Yi Yun's embryo. First off, she still hadn't made any contact with Yi Yun whatsoever. Also, she just seems to genuinely want a baby since she could be or her husband might be infertile.
I suppose the problem would be that they won't be as good as parents to Yi Yun's offspring so... MORE DRAMA TO COME ヾ(❀╹◡╹)ノ~