Kylus being one of the possible love interest is a big no no for me

Omy I hope his and her grace are alive throughout the whole story. I hope nothing bad happens to them. I want to see more of them bonding together as a family. It seems Kylus will be one of the MC's love interest. That's what the story is portraying so far. That to me is a bit wrong? I don't know how to put it but why can't adopted daughters just have a loving family with no brothers that are trying to be with their adopted sister romantically?