That's right Akira! You tell Ayako. What is wrong with those 2 being together??? They can't reproduce, and Asuka seems to be the only human Ayako has ever felt something for. Soul mates. (shrugs)
Yes. It's probably going to be a different slant if this were heterosexual (father x daughter), with the possibility of the minor aged girl getting pregnant.
Asuka's not a "child" in the sense that she's a toddler or someone pre-adolescent. Depending on the laws, she's "minor aged", but definitely not a bumbling child who can't informatively consent on her own.
Situational ethics. If I'd known a mother-daughter 'couple' like Ayako and Asuka, who have the same circumstance as theirs, it's not ethical to call the police. I'd let them be. They wouldn't affect me in any way.