Lady Middleton is described as cold and insipid, and I've always thought of her as rather harmless, if dull. This read I noticed more how she drops none-too-subtle hints.

I've always noticed (and laughed) at this one:
" As it was impossible however now to prevent [the Steeles] coming, Lady Middleton resigned herself to the idea of it with all the philosophy of a well-bred woman, contenting herself with merely giving her husband a gentle reprimand on the subject five or six times every day."

and this one:

"I am glad," said Lady Middleton to Lucy, "you are not going to finish poor little Annamaria's basket this evening; for I am sure it must hurt your eyes to work fillagree by candlelight. And we will make the dear little love some amends for her disappointment to-morrow, and then I hope she will not much mind it."

This read, however, I made a connection between this exchange between Lady M. and Sir John in Ch. 20:

When they were seated in the dining room, Sir John observed with regret that they were only eight altogether.

"My dear," said he to his lady, "it is very provoking that we should be so few. Why did not you ask the Gilberts to come to us to-day?"

"Did not I tell you, Sir John, when you spoke to me about it before, that it could not be done? They dined with us last."

and this bit from the chapter before:

' Sir John had been very urgent with them all to spend the next day at the park. Mrs. Dashwood, who did not chuse to dine with them oftener than they dined at the cottage, absolutely refused on her own account'.

At first I was thinking that when Mrs. Dashwood chose to 'stand upon ceremony' in that way, it sounded far less harsh than Lady Middleton doing the same thing. Then I was wondering if perhaps Lady Middleton had dropped one of her subtle hints to Mrs. Dashwood about the reciprocal invitations?