I agree that there is some competitiveness in Mrs. Norris' motivation. After all, she was Miss Ward and she found herself making way to a younger sister as Lady Bertram. I really do think sibling rivalry is still going strong with her, but she's found a way of twisting it around so that she's still bossing her little sis around, even though little sis has risen above her in rank.

However, I'm seeing something in this reading that I've never seen before. I really think that boredom is causing Mrs. Norris' imagination to run away with her and that this is at least part of what drives her.

I was struck at first by her not being able to get poor Fanny's family "out of her head". Much as I hate to admit such a thing with Mrs. Norris ;-) that's a very human thing. And in that situation, it's natural to wonder what you can do to help. What lotto-player among us hasn't thought of sending a child to college, giving food and new clothing to people who need it or otherwise helping the needy with an imaginary windfall?

So somewhere in her imagination, Mrs. Norris comes up with this idea to help. However, she's not generous enough--with either money or her own personal space--to actually do it herself. Instead, she coaxes the Bertrams into it, gets to do some busy work--thus easing her own boredom--and gets the credit for having done the good deed. Not bad for the sister who married second-best, eh?

I've found other instances of Mrs. Norris' imagination at work in the first couple of chapters and I'm going to keep an eye out for more. One of them is this:

A niece of ours, Sir Thomas, I may say, or at least of yours, would not grow up in this neighbourhood without many advantages. I don’t say she would be so handsome as her cousins. I dare say she would not; but she would be introduced into the society of this country under such very favourable circumstances as, in all human probability, would get her a creditable establishment.

So Mrs. Norris has never met Fanny and already she's imagining that a)Fanny's not as pretty as Maria and Julia and b)her association with Mansfield Park is going to help her make a favorable match. That's quite a jump from, "Let's take our niece in and give our sister one less mouth to feed."

Immediately afterward, she says to Sir T, regarding the cousins-in-love issue:

Suppose her a pretty girl, and seen by Tom or Edmund for the first time seven years hence, and I dare say there would be mischief. The very idea of her having been suffered to grow up at a distance from us all in poverty and neglect, would be enough to make either of the dear, sweet–tempered boys in love with her.

Again, either this is coming from a vivid imagination or she's been reading romantic novels. ;-)

Later, in Chapter 4, Mrs. N's imagination comes into play again when she already has planned out how she's going to tell the others that Sir T's been killed on his journey. The same is true of her later thoughts that Sir T's sending Tom home ahead of him is proof of some sort of dire premonition. Really, the more I read, the more I think that Mrs. Norris spends quite a bit of her time imagining and daydreaming.