Your last sentence sums Mary up pretty well IMO.

I think what irks me most of all - the final straw, so to speak -is her intellectual dishonesty. She is self centred and inconsiderate, which is bad enough, but she takes it a step further by pretending she is caring and helpful. She trumpets her concern for her "dear little boy" whilst swanning off to a dinner party. In Lyme she preens herself on being "so very useful", when she is in fact indulging in hysterics and enjoying a seaside holiday whilst leaving all the donkey work to Mrs Harville. I might think a little less ill of her if she could admit, if only to herself, that Mary's number one priority is Mary.