I think with both Alma Jr. and Ennis we're supposed to perceive something along the lines of the old saying "still waters run deep." My sense, is that the audience is meant to see both of them as very observant and smart in an introspective way (while neither is probably particularly well-educated in a formal/ official way). I think we're also meant to see them as having some unspoken forms of communication between themselves (they way they're shown talking in Ennis's truck, etc.). And, again, I think how an audience member views their modes of personal communication greatly impacts how the last scene of the movie is interpreted.
I do agree with the part of delalluvia's post suggesting that Junior's happiness at hearing that Ennis will quit his job in order to attend her wedding is a bit self-absorbed. It honestly really is a lot to ask of a person who is impoverished. It's not too much to hope/ expect that her own father would be at her wedding, of course. But, the point here is that she's happy he's quiting a job to do this (that aspect of the situation seems a bit extreme). Still, maybe she's been raised to expect Ennis to quit jobs at the drop of a hat (since he clearly did this a lot in preparation for meetings with Jack).
Her glee at her own new-found love and upcoming, sanctioned heterosexual union also comes across as hard to take (for me personally as a viewer) knowing what Ennis has had to struggle with in his relationship with Jack. Of course, Junior, probably doesn't understand the full context of all of this... so I don't hold it against her. But, I always think the moment when Ennis looks to the side and pauses a bit during the conversation and almost seems to tear up, probably has a lot to do with him contrasting his own situation with Jack to the circumstances now surrounding his 19 year old daughter at the beginning of her own relationship.