George's Sister

if his sister had not done the telling, then it would have had to have been done by Serena herself. I say that because of the many vague spots throughout this movie. I doubt if the producers would have wanted Serena to spend time telling Pemberton about her childhood. It seemed to be presented as a fairy tale romance at beginning even after the sister scene.
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Just try to stay alive and see what the next minute brings.

I was just wondering why the sister never appeared in the film again. Maybe her other scenes were cut and only the scene most pivotal to the plot was retained.

For me, the fact that she was only seen in that one scene made it obvious what her function was.

Well, he'd traveled home to Boston to visit her, for one thing - did you expect her to go visit them, something like that? Also, having Serena's backstory told by someone else was, I think, necessary, as it gave her more of a mystique, and it didn't seem in her character to have told him much of anything, herself.

... have her appear once for the sole purpose of telling George Selena's backstory.