The relationship between sisters sure does go through changes as they grow up. Mary and Lydia's relationship is shifting in this story -- especially in Chapter 7 from where the excerpt below is taken. It's a bit of a role reversal as the little sister is stepping up to care for the big sister.
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Lydia threw her arms around Mary and pulled her into a tight hug. “Oh, Mary! Being alone in the middle must be dreadful! How you must suffer for it. I will not allow it to be any longer,” she said, and then, she did something she had not done since they were both very young. She kissed Mary’s cheek.
[from Persuading Miss Mary, book 4 in the Marrying Elizabeth Series]
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Confounding Caroline ~ Delighting Mrs. Bennet ~ Loving Lydia