He just watched his father's casket carried by him. Grieving? Obviously. "Stunned"? Yeah, OK.
Happy New Year
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Ah, I realize I've seen the analysis of the guests (video) in context and my comments are posted out of context.
They were in grief mode, the casket began to move past them. The SS handed George the envelope, he glanced at it and handed it to Laura, her faced changed after she looked at the letter (some say it's a photo as they don't look at it long). She's shocked and gasps:
Note Jeb has a smile of acceptance/grief on his face. That expression goes away completely after Laura shows him the letter.