Here's a fun thread I never saw before.
I agree that a twitch implies something reflexive, involuntary. I suppose it could mean any of the things mentioned above, but I'd vote for shock.
Another great example of Annie's use of ambiguous phrases that makes the reader do the work.
Regarding Ennis's feelings towards his daughters, there is one harsh line that occurs later, after the Thanksgiving debacle:
He didn't try to see his girls for a long time, figuring they would look him up when they got the sense and years to move out from Alma.