The Ballance isn't right on this one. It's definitely trying to build a romance between the two now stepsiblings, but instead it's building a dynamic where the older sister really just wants to help her new brother through his trauma and become a good kid who goes to school normally while the brother seems like he's trying to make out with his sister constantly because that's the only way he's ever known love. Honestly it's better set up for them to end up proper siblings in the end and have healthy romantic relationships with other people. But it's almost guarantee that won't happen.
By chapter 25 she is still saying her brother smells like a baby, thinking of his trauma and thinking of another guy in romantic interactions, while the brother is just destroying her chances of making any friends constantly and not understanding love. He just screams trauma and I can't feel any fluffiness with their romance. Cheeky brat just did it better