I'm not really sure. I think if David was phase 1 then Sterling would have been in on it. Remember Hunter really doesn't have a problem with Sterling seducing the family he only really has a problem with the fact Sterling manages keeps cockblocking him. Sterling only left because David was afraid he'd turn into another Hunter. I don't think Sterling would have ever left the house if Hunter was in on it. Hunter would just train him to help him seduce the family
Keep in mind that Sterling was away for the last
four years, only returning
because David had died. Hunter or Drew might have put their plan into action
after Sterling left. We only know that David had an affair with Drew for (an unspecified number of) "years", but I'm speculating that affair began while Sterling was away. Hunter and Drew may not have predicted that David's death would trigger Sterling's return, but they probably expected an easy conquest whether Sterling was there or not.
On second thought, it's conceivable that the affair started while Sterling was still in the house. If that's true then David's harsh treatment of his son could have been partly due to guilt and psychological projection, not just fear of Sterling seducing his family.
Also I'm only assuming that Hunter and Drew have been coordinating their schemes, but it's quite possible they act independently or have different motivations. It would be interesting to learn more about their true intentions.
In any case I enjoy this theorizing and discussion, and I'm fine if my guesses turn out to be wrong, because there's still a lot we don't know. The mystery of who-knew-what, and when-did-they-know-it, is a big part of what makes this story compelling to me. I expect more unforeseen plot twists and I can't wait.
