I'm surprised to see these comments still. It's very clear what the endgame is; have the dad fuck the daughter to get her pregnant. Why? Well, it's not super clear, but based on the conversations the mom/daughter pair has had, it feels as though this is a plan to re-create something or someone; making 'papa' come back to the pair's lives.
Right now, we're seeing their long plan to isolate him from his family, especially his sister, come to fruition. His sister has been protective of him and was suspicious of them, but now she sees the mom of the pair as an ally and has set off the careful bomb planted by the pair.