Agreed. There's no need to just turn one into a villain to make readers hate on one and love the other. I hope for something more nuanced, deep and complex than that. After all, the other child is also a victim who had no control over this, and who can let go of the parents they love instantly after hearing they're not biological? If she could let go of them instantly I'd have questions about the bonds between them. Blood relations doesn't mean you'll love, support, and take care of each other, and lack of blood relations also don't mean you can't love, support, and take care of each other. I like Yao-shi because she insists on trying to properly discuss it with the individuals involved and listening and respecting the wishes of those involved.

I suggest you try the 'Atypical Young Lady Returns'. Twin sisters get kidnapped at early age. Only the younger child is rescued by the parents. The other was missing until she was about 17. The dynamic between the two sisters is an evolving thing but I think you would enjoy the depth of it.