I actually had the same thought, which is surprising as I am not a big fan of the menage a trois trope. But, it really would have been beautiful to see.
The guy doesn't like the twin sister that way though. Forcing the guy to be with both the twins when he doesn't have feelings for one of them is bad for both the unrequited twin and the guy(it's like rape.) It's better this way since the guy does not have those feelings for the girl. Don't force the guy to be with someone he doesn't like that way.
Just because the girl gets an unrequited crush, it doesn't mean it's ok to say "hey he should date them both at the same time even if he has no feelings whatsoever for the girl" Chapter 7 basically showed that the girl never had a chance.
I know this isn't very fujoshi of me, but this so should have become a polyamorous relationship. With the twins that close, it would have been nice for them to have a mutual lover.