One thing I'm pretty sure is that Alma Jr. loves his father very much. The disapproval look on her face when she saw dad's with a new girl, well, I'd say it's a normal reaction of a teen who just wants to be with her dear daddy, alone.
Her reactions with Cassie while in the bar, well, I'd say Alma Jr. was quite "polite". "Polite" as in that's her character, not because she knew her father liked/loved a man. If it's any other films(aka "straight movies"), it can pretty much end with a spoiled teen going all cranky and angry and teary and...