Here's another example of Mary's almost pathological jealousy from Ch12, after Anne's second encounter with Mr Elliot

At the same time, however, it was a secret gratification to herself to have seen her cousin, and to know that the future owner of Kellynch was undoubtedly a gentleman, and had an air of good sense. She would not, upon any account, mention her having met with him the second time; luckily Mary did not much attend to their having passed close by him in their early walk, but she would have felt quite ill-used by Anne's having actually run against him in the passage, and received his very polite excuses, while she had never been near him at all; no, that cousinly little interview must remain a perfect secret.