the whole annie thing works for a couple reasons. 1 sara asserts her dominance over annie making it clear everything with her daddy is with her permission only, which has been everything she worked for since oh sara. she was never against sharing him, but his heart had to be hers. 2. sara and daddy need help and allies they can trust to uncover the truth and make themselves safe, us against the world doesnt become any less so but rewarding a loyal friend with some daddy d. 3. like part 1, daddy has always been fucking women, but now sara has the reigns and this scene shows that. she just cant get him to seed her yet, but we know she gets what she wants in the end.
i know a lot of people dont like annie and thats okay, just saying i dont really feel that it goes against the story, rather reenforces it. to top it all off, this story always stood out above the others cause the driving factor has always been sara. it was never a story about a dad corrupting his daughter. its about a daddys girl corrupting her daddy and convincing him to belong to her in every way, above everyone else, even her mom.