A lot of you say that manga authors hate communication, but I think in general people in real life are the same way, which is why so many marriages end up in divorce or both partners having to put up with each other.
This is especially true for high school aged cousins who would be committing incest if they get into a relationship. It's a melancholic ending, but that's how the cookie crumbles most of the time, and Murasakino loves writing about it. Let's just be glad that if they were to start dating, they aren't able to make babies lol.