I was contemplating a scenario where Hunter threatens to hurt/kill Sterling unless the girls comply with his demands. The problem is they'd probably be angry/crying as they are forced to have sex, and that just doesn't sound hot to me.
Hunter's got to get the girls to want it somehow, which is going to be difficult now that they know what he's been up to and he's out of the house. I think he's going to need an accomplice or Trojan horse to get past the family's defenses and create an opportunity for him.