@kuma as far as I know it's somewhat common for victims of abuse to "inherit" those same patterns of abusive behaviors, especially if they're young. We know that both the MC and her brother were abused by "papa" but we don't know everything that "papa" did to the brother aside from hitting him. So if the brother saw what was being done to his sister, or it was done to him as well, then there's the possibility that he might think any number of things that make sense to him to start that behavior with her. Not to mention he's older and probably going through puberty so sexual urges are a new experience for him, and after their fucked up childhoods, he might just think it's normal or good. But there's clearly a lot of damage to him.
Not saying it's good behavior or that the brother is a good person, but it's not out of nowhere, and probably not his fault initially. Though based off where the story has been and probably will go, I have a feeling he's going to do some pretty terrible things. This all is just making me feel really bad for Nazuna more than anything...