I think it's a bit of a win/win situation as far as Hunter is concerned. If he can push the MC to fuck his family, he proves he's right, which is a win in his mind (and will probably try to force the MC to share because he's the one who showed him the way). But his original plan is of course to fuck them himself, and the reason why he wants the MC there is to prove his superiority. He wants the MC to know he's powerless to stop him. It's part of his powertrip. It's one thing to fuck a married woman, it's a whole other thing to fuck a married woman in front of her husband with him unable to do anything about it.
Also, I think there's some things in that will that will paint the MC's father in another light, and Hunter knows about it. I wouldn't be surprised if the two were close in the past, but the father shut him out when he married Hunter's daughter. Could also be the reason why he was so harsh on his son because he was afraid there was some of Hunter in him.