This is a pretty broad question, especially considering that Lady Seymour was always doing something new and rather outrageous, at least as seen through Isabel's eyes. In Chains by Laurie Halse Anderson, Lady Seymour owns Isabel, a young black woman, though we know that Isabel really should be free. Her freedom was taken away from her by the unscrupulous and greedy nephew of the woman for whom Isabel worked--the same woman who set Isabel free.
Lady Seymour, on the other hand, is a cruel, supercilious white woman who simply acts and reacts as the mood strikes her. More than that, she and her husband are keeping secrets about their patriotic allegiances, and both of them are rather desperate to keep those secrets. Because I do not have a specific chapter number or incident to work with, I can only make an educated guess what incident you might be referring to in your question.
In chapter four of the novel, the Stocktons, Isabel and Isabel's sister Ruth are on board a ship heading to New York. When they get off the ship, one of the officials insists on examining the Stocktons' trunks, accusing them of being supporters of the King (Tories) and therefore spies. Lady Stockton poses no objections when the officer, Bellingham, wants to search her husband's luggage; however, when he wants to search her ornate walnut trunk, she throws an absolute fit.
Lady Stockton's objections are so strenuous that they cause great astonishment to nearly everyone who is near them. She not only speaks loudly, but she insists that Bellingham--or any other gentleman, for that matter--will certainly not be allowed to put his hands on her undergarments. Bellingham is embarrassed and others are shocked at this exchange. Isabel, on the other hand, is rather amused, though she is savvy enough to keep from showing her feelings. Unfortunately, her younger sister Ruth laughs out loud at the rather ridiculous exchange.
This infuriates Lady Stockton and she demands to know who was laughing at her. Immediately Isabel steps in to protect her sister and takes the blame, and Lady Stockton's reaction is quick and sure: she slaps Isabel hard across the face. It is an astonishing thing to Isabel and, frankly, to others who see it happen. All Isabel can think is this:
Better me than Ruth, better me than Ruth.
This incident just confirms what Isabel already knew, that lady Stockton is not a nice woman, and Isabel and Ruth are both going to have to be careful if they want to avoid trouble.