It is possible that Lady Bertram is actually a zombie. We're not entirely sure that she's fully aware, awake, or alive half the time. Lady Bertram is so lazy that she comes across as constantly out of it.
Her own family is aware of this and tends to either ignore her, make fun of her, or have conversations right in front of her without caring about her overhearing anything. Even the always-serious Edmund gets a laugh out of his mom's oblivious attitude when Tom tries to convince him to go along with the play scheme:
"It is a very anxious period for her."
As he said this, each looked toward their mother. Lady Bertram, sunk back in one corner of the sofa, the picture of health, wealth, ease, and tranquility, was just falling into a gentle doze [...].
Edmund smiled and shook his head.
"By Jove! this won't do," cried Tom, throwing himself into a chair with a hearty laugh. "To be sure my dear mother, your anxiety – I was unlucky there."
"What is the matter," asked her Ladyship, in the heavy tone of one half-roused; – "I was not asleep." (13.19-23)
"Half-roused" from sleep seems to be Lady Bertram's default state. She's a fairly ridiculous figure, and is often highly "convenient" too.
To her more rebellious kids, Lady Bertram is the best mom ever. With her in the room they have nominal (as in, in name only) parental supervision without any of the consequences of actual parental supervision.
Neighbor Henry finds Lady Bertram rather convenient as well. Seizing an opportunity to speak to Fanny, Henry notes that "Lady Bertram's being just on the other side of the table was a trifle, for she might always be considered as only half awake" (34.40).
Lady Bertram's total lack of awareness extends even to her own opinions. She constantly has to ask Sir Thomas what she should think about certain things, and her husband and her sons pretty much tell her what to do at all times. Overall, Lady Bertram is a very negative figure of wealth: lazy, spoiled, barely even aware of the nice house and lifestyle that she has.