I think a lot of the people here complaining about Sterling and Ophelia not learning lessons and having personality shifts need to read the dialogue again. To my mind there is something going on with Sterling, something "Hunter'ish" add to the fact Hunter is drugging everyone in the entire house with god knows what that affects them god knows how.
In ch 9 he steps up his game and starts to drug the very air everyone in the house breathes.
So Ophelia having gone through the loss of her husband, his debts, their children, Hunter's "help" and her body acting crazy, now Sterling crosses the line, high on whatever aphrodisiac that Hunter sprays them with and bring Ophelia well over her comfort zone and basically mouth-rapes her.
Of course it will send her over the edge, in the book club scenes she eventually admits that she wants to experience control in the bedroom, being dominant, and now with what happened between her and Sterling and she knowing that she needs this as much as he wants it, she decides that to continue their after dark activities, she needs to play a character that she has always wanted to play.
It's not at all a leap in logic imo what happens between several of the characters this chapter.