I remember there was something about blackmail. But think about whether he will add blackmail again. And Ellie is definitely not atracted to her brother, and behaves like a bitch with him. A dramatic change of character will not be peculiar, except with a good scenario. Remember, she loves older and successful guys, and for her, Dylan is a loser.
Well she sure is very harsh towards Dylan even without any reason, while she is generaly nice to evetyone else.
In my experience that means she actualy cares about him. And it makes perfect sense - for a sister.
She doenst actualy hate him or despise him. It is more like she is hard on him trying to make him into a better person. I have a 1,5 year older sister and it is exacly like that here ;] (no incest tho ;p)
If she didnt care about him she would simply ignore him instead I think.
If you wanted to make their incestous story true - I would use that in their character development.
I would write that Ellie started dating older popular guys exacly becouse she could not cope with herself beeing atracted to her brother. In order to forget about him she would date guys that are compleate oposite of him instead. But she would never be happy with any of the jocks, and her frustraction would grow with time.
Eventualy, in proper circumstances, with a mixture of lust and love, shame, excitment and embarasment, the right conditions to make their relationship a phisical one might occur.
It might be hard to pull off inside this game however - with so many other plotlines interfering...