Responses
yes. It was implied quite strongly that Takahiro's first love is his sister. Yes. It's the sister's ring. That's why he was sad when he lost his memories and saw the baby again. It's ok if you don't 'do' incest, but it was quite obvious throughout the story, they can't be more in your face about it without spelling it outright for you. I understand people have their own squicks but don't let that stop you from reading the story objectively.
I cant believe ive gone this far into the manga without knowing, but who's ring is it that takahiro takes? Is it his sister's ring?