So they want him to get married with the sister, but she's not actually his sister. But then they proceed to raise them like brother and sister???.... If there's a horrible parents award these two would be winner no.2 with no.1 being actual abuser that directly hurts children.
I mean that's one way to integrate incest in a manga. Maybe the mangaka wanted to make one but didn't wanna take the backlash (if there's even anything like that in japan).