Damn, I had an expectation that they'd both snap out of it, but the moment it should have happened, the moment the author clearly set it up to happen, the boyfriend went and doubled down on it. The situation they're in is pretty unhealthy all things considered. They'll eventually have to face the truth, but are they going to end up falling for each other? The boyfriend clinging to the image of his dead girlfriend, and the brother clinging to the one person who could help him feel closer to his sister? It's a weird situation to be sure.
I'd like to think both of them will be able to use their connection to the deceased to end up in a healthy relationship, whatever form that takes, as friends, acquaintances, or more. Without the need for the illusion.
Part of me also had the expectation that would be revealed the boyfriend wasn't actually the boyfriend after all. Kinda glad that didn't happen.