If you cannot grasp the moral centre of the movie then of course you won't like it. It was perfectly clear to me what emotional strings were being played, and for hundreds of other people too. You don't need to like the movie, but criticising people for crying at it? I'll contend that's quite ridiculous, especially considering that it's quite a moving piece.
She wanted to Atone because she realised how her childish misunderstanding had deprived her sister of her love, the focus lies there. Dismissing the atonement as pointless because she was a silly unaware child is circular thinking, for the rub is that she was silly and jealous and as an adult she now regrets it.
I thought the ending was a little strange, and perhaps played on the sentimentality you dislike so much, but it's required to give the plot twist an emotional force.
In short, I don't understand your criticism at all.
