I have always found the two Thanksgiving Day scenes to be reflective of very important ideas in the development of the two main characters, as well as clarification of some minor ones.
We learn a lot about Jack -- he DOES have a breaking point. In so many other scenes, he seems to keep quiet and declines arguing, but here we see him fed up and determined to have his way.
We learn about Ennis when Alma confronts him with her proof that he never went fishing with Jack. He knows, now, that Alma knows what he and Jack did on their trips, and he has to make a decision, quickly, about how to respond. Anger wins and he lashes out at Alma, telling us that he can be pushed too far, especially when it comes to being outed.
We learn that Lureen's old man is a despotic old martinet, a real stud duck.
We learn Lureen's mother is completely cowed by her husband so she never said a word.
Lureen is proud when Jack stands up to her father, indicating that
she did care about Jack, and, maybe one reason she married Jack was to get away from under the old man's thumb.
We get a glimpse of Bobby as a puling little kid, not too happy he didn't get his way. He didn't seem too close to either parent, or grand-parent.
At Alma's house, we see Alma in her nice house with her new husband, but we don't see her happy. She may be content, but she doesn't smile, looks down all the time, shows no signs of happiness.
Her husband says NOTHING -- All we know about him is that he allowed Ennis to share Thanksgiving dinner with the family. Other than that, he is a cypher.
We know the girls are well-behaved and happy to see their father, but they, too, show no obvious signs of being happy.
And Ennis is just so uncomfortable, you can feel him wanting to crawl out of his skin and get the hell out of there, even before the dish-washing scene.
These two scenes are important to me because of what they show us about the characters, as well as about ourselves. I'll stop here and see if anyone has discovered more within them.