I don't know what flags you have to hit with what characters but I know there is a check box in the options of the game and is by default turned off. So turning that on would be the first step. It is in Menu>tags>incest(check mark it)>then confirm it.
At least Vivi (the sucubo) is the MC cousin/sister, because when talking about magic she says ¨I don´t expect less from my blood¨ refering to the MC. I´m still a bit confused about this but Lucinda could be their aunt, or just nanny.
