I wouldn't say that Haruyo and Kasumi are pretending or playing a role, as much as they are allowing themselves to be something else than what they are.
I do agree in Haruyo's case but not entirely in Kasumi's. I think the way both engage with their "roles" is a bit different. Haruyo is able to be the person she wishes she were when playing the "onee-sama's" persona. And it is through "onee-sama" that she might become that person, a person who's words are being taken seriously.
With Kasumi though, her role as the "imouto" feels more like the real Kasumi. Haruyo has to make an effort to become "onee-sama", while Kasumi hasn't. She has always harbored those feelings of wanting to be pampered and being told everything will be okay, but that's all repressed and closed off inside of her.
The S kankei allows Kasumi to be live closer to the person she is, while it allows Haruyo to live closer to the person she wishes she were. It still may be just semantics, but I've always had this thought nagging me about how there was a missmatch in the perception they had about each other, and how Kasumi wanted "onee-sama" more than she wanted "Haruyo" but Haruyo wanted "Sumi-chan" which is pretty much just "Kasumi".
Since Kasumi now knows more about "real" Haruyo their dynamic has changed quite a bit, so it doesn't really apply anymore. Slowly, their "personas" and "true selves" are starting to blur more and more, which makes it difficult for them to dismiss any kind of affection as just playing pretend, and they surely are aware of that.