In the Hunter/Brenna guitar scene, and the Hunter/Catherine gym scene, I guess Hunter
could have seduced them naturally, but I think Hunter's drugs likely played some part. He'd probably been in the house long enough to tamper with their food/drink, hygiene products, etc.
Come to think of it, I'm not sure how long Hunter was living there before Sterling came back. Hunter says Ophelia contacted him for help with the mortgage, which would have been
after David's recent death. His funeral was only the day before Sterling's return, so how long could Hunter have been there realistically, maybe a week? But even a week seems like plenty of time for Hunter's drugs to work their magic, making the ladies horny and lowering their inhibitions.