I'm not sure about how much she perceived of her father's feelings for Jack. We really didn't see enough of her to really know.
I disagree.
When I try to look at the screenwriters’ choices, it reinforces my belief that Junior knew.
I often wonder about the storytellers’ motivations for choosing this-or-that dialogue. Is it a part of the plot, or is it character development, or is it a red herring, or is it simply a mistake; an extraneous bit that never managed to get edited out but in the end contributes nothing.
In this film, nearly every detail seems to contribute something of value.
And what would be the value of Junior being jealous of Cassie?
It isn’t important to the story.
She *is* jealous, of course, but that’s not a valuable piece of the plot here; she’s portrayed as such simply because it’s fitting—she’s not getting private time with her father.
But to me the point of the scene is to show her connection to him. She observes him quietly. She's concerned, and she's wondering. She strongly suspects the truth about him, even though he's not the 'type' at all.
To me, there’s only one way the writers would include the scene with Junior and Cassie. It’s not to show that Junior is jealous (most children of divorce are, under such circumstances) or that Cassie is pushy or Ennis can’t dance (we knew these things). It’s to show that Junior knows (or will, by the end of the trailer scene). As Cassie drags Ennis to the dance floor, she’s studying them because she’s questioning his sexuality and trying to understand.
We see this because we’re meant to sense her growing awareness about her father. Isn’t that the reason writers chose to show the deep connection between father and daughter? Why would they do this, except to suggest Junior’s awareness of her father’s secrets?