I dont think thats going to work as you can see the black hair boy strongly opposed the adoption idea and It is crystal clear to us both the Fl and balck hair boy have feeling for each other if this actually go the way you want it might turn disgusting and unbearable to read ;)(≧∀≦)
At first I thought this way too but as the story progress they we'rent really giving siblings energy. Tho I think there may be some issues with the duke in the future if they did move forward with their relationship.
The story's fine but I'd prefer the fl and the black hair dude (ml?) As brother and sister bond tbh