I'd like to chime in with Edith Lank and say that if one goes through Lady Bertram's statements in MP one finds them to be anything but silly. She may be lazy, unashamedly selfish, and indifferent to what's happening around her as long as it doesn't threaten her or what she cares about, but she's not unobservant nor does she fail to notice what is in her interest, nor how her actions may be interpreted, nor what are the motives of others. Think of Mrs Bennet and how her remarks are often long-winded, not in her interest, and blind to what's going on about her.
After Lady Bertram's comment, "'I think we cannot do better... let us send for the child'" (I:1, 5), her next is "'Then she had better come to us,' said Lady Bertram with the utmost composure" (I:1, 9). This is in response to Mrs Norris's long-winded and series of phony excuses; it's short, to the point, and "composed." Lady Bertram is not surprised by her sister. We may not like Lady Bertram for saying in defense of Fanny that she sees in harm in the child and she's very handy for one's errands, but it's not wrong. It's not a misinterpretation. It's unfeeling towards Fanny, but not dull. In fact in the descriptions by the narrator which are carefully flat and neutral there is nothing to the effect that Lady Bertram lacks sense. What Lady Bertram does not want to do is "put herself to any inconvenience" (I:2, 20). Note that in the dialogue Edith quoted Mrs Norris is alive to the sting in Lady Bertram's response to Mrs Norris's hope she will be able to "lay by a little at the end of the year. 'I dare say you will. You always do, don't you'." Mrs Norris does not think that's a chance hit, something Lady Bertram said by mistake, but defends herself and appeals to her sister's interests by saying her "object ... is to be of use to those who come after me. It is for your children..." (I:III, 30). Mrs Norris does not underestimate her sister's understanding.
To move a bit forward to the scene in Chapter VII when Edmund comes home from dining at the parsonage to discover Fanny has an headache, it is clear that Mrs Norris is aware her conduct could be blamed for Fanny's headache. She is overlong in her excuses for herself, she tries to become deaf, she turns about, she then moves to needle Edmund precisely where she knows he has been to blame by responding to his comments about her demanding Fanny walk back and forth for her by referring with what appears to be no personal reference to Fanny's not having ridden the horse for a while. Edmund gets the point. Lady Bertram's equally astute observation goes unnoticed because she says so little, gives so little away. She's also better at appearing not to be aware she is making hits. Edmund is going on about the roses, and suddenly, just when Mrs Norris says maybe you sister could offer Fanny your "aromatic vinegar; I always forget to have mine filled'," she blurts out, as if unconscious and thoughtlessly, "'She has got it... she has had it ever since she came back from your house the second time'" (I:7, 73), and thus is Edmund's attention deflected from her abuse of Fanny to Mrs Norris's. She later does not deny Fanny perhaps got a headache cutting flowers; she admits it freely--just as Mary Crawford freely admits to Fanny it was pure selfishness that made her keep the pony from Fanny. It's a great technique, telling the simple truth. It defeats people. They have nothing to offer against you. Mrs Bennet is a baby in comparison to Lady Bertram.
Do not underestimate Lady Bertram's awareness or understanding of what she's about. Very late in the book her advice to Fanny is to marry Crawford because one should marry money. Well look at her. Let us recall it was she who captivated Sir Thomas. Her uncle thought she was three thousand short of an equitable claim to Sir Thomas's fortune, but maybe he too underestimated her abilities.
And who in this book would say her one piece of serious advice to Fanny--to marry money (=Mr Crawford) is bad advice. Certainly not Mrs Norris--nor Mary Crawford, nor any of them. Even Fanny's not against money; she just wants to love the man she marries first, and has long ago learnt to love someone else. Otherwise think of all the things she could have done for Susan.
To this Dorothy Willis replied:
Subject: Impetuous Lady Bertram
I'm not making a joke with this subject line. I had not noticed before that it was Lady Bertram who actually forced the decision to have Fanny come. thanks, Ellen!
We'll have to watch for other instances. Maybe she is not such a brainless blob as she appears.
Dorothy Gannon also commented as follows:
Now that you bring it up, Ellen, I'm seeing for the first time the criss-cross of sisterly jealousy that filters back and forth between the older women. Mrs Price has long ago given up any pride: she can't afford to be jealous; but Lady Bertram and Mrs Norris are always obliquely going at each other. Lady Bertram, with a well-placed nudge now and then, Mrs Norris always compulsively jabbing.
Yes [now talking about later in the book] Lady Bertram must have gotten sick and tired of the constant harping Mrs Norris was doing about how cleverly she had forwarded the match. Lady Bertram is lazy, and her sister does take care of lots of little bothersome details-- but, having known her sister since birth, she probably has a pretty good idea of just how much loving-kindness lies between all that bustling activity.