In ch.5, it says:

Being alone, [Mary] being unwell and out of spirits, was almost a matter of course. Though better endowed than the elder sister, Mary had not Anne's understanding or temper. While well, and happy, and properly attended to, she had great good humour and excellent spirits; but any indisposition sunk her completely. She had no resources for solitude; and inheriting a considerable share of the Elliot self-importance, was very prone to add to every other distress that of fancying herself neglected and ill-used. In person, she was inferior to both sisters, and had, even in her bloom, only reached the dignity of being "a fine girl."