It's obviously impossible to guess where the "plot" is going, but in terms of at least trying to justify getting the MC and his mother together, it would make much more sense for Ella to be unavailable to the MC because she's actually Daddy's girl. Mom sees that, gets jealous, revenge-bangs son.
Meanwhile, the still-unseen sister would rather obviously be the villain (and the photo-taker) in a normal version of this story. What will actually happen? Who knows?
I think everyone would be a lot happier just treating this as a pure porn game rather than trying to make sense out of it. Because otherwise you find yourself trying to rationalize the fact that your MC's a serial rapist (when he's not being incredibly creepy).