That's an intriguing reveal.
There was that chapter earlier when they were on their date and he saw an ad featuring "Akarin" and it seemed then that he suddenly figured out her secret, but then decided to just leave it alone. But I was never sure that that was exactly what happened, so haven't been sure whether his later references to her sister were sincere or just him playing along and keeping her secret for her.
And presumably she figured out, but isn't going to tell him, that she was jealous.
More progress in 8 chapters of this than 100 chapters of most.