I voted "who can tell" because really, who can tell? There is no set way to act if you've been abused and there is no set type of abuser. And this family certainly has some of the most complicated and IMO sick dynamics of any I've seen and I come from a strange group myself. I'm still trying to get past a brother being so upset that he isn't "included" in his sister's unplanned pregnancy-sorry, to me that is just plain weird. Included in what way? Did he want to pick out the wallpaper? I grew up with 4 brothers, and this just kind of blows my mind, I guess. In my world, older brothers in their mid-twenties go about their own business and lives.
Casey's behavior toward her father does not preclude, to me, the possibility that there was sexual abuse. I could see her, if it happened, holding it over him for the rest of his life, running the show, etc...and yet still throwing him bones, I love you, you're the greatest, etc...
It's not like you have to stop and remember that this isn't a "normal" family-there is nothing about them to make you ever stray into thinking that they are...Casey didn't become Casey without the "help" of whatever percolated inside that household.