
This is incest ??

Yes, they’re half-siblings. Father is the same. Father was an asshole who cheated on his wife with a sex worker. Sex worker decided to have the child because the father had MONEY.
(Side note: A percentage of Japanese people do believe it’s not cheating to sleep with a sex worker when in a relationship. Ugh, crazy scumbags.)
Why couldn’t she just say that’s her mom’s ex husband and biological father ? Characters be so allergic to communication an cause so many problems