Yes, I was just about to post on this. This is something we've discussed in previous group reads of S&S. Roger Gard, in his book Jane Austen's Novels: The Art of Clarity gives quite a thorough explanation of the similarities between Fanny talking John down to giving his sisters nothing and Regan and Goneril reduce their father's retinue down to nothing as well.

Here is the passage from Act II scene iv of King Lear:

KING LEAR

I can be patient; I can stay with Regan,
I and my hundred knights.

REGAN
Not altogether so:
I look'd not for you yet, nor [now] am provided
For your fit welcome. ...

KING LEAR
Is this well spoken?

REGAN
I dare avouch it, sir: what, fifty followers?
Is it not well? What should you need of more?...
Yea, or so many, sith that both charge and danger
Speak 'gainst so great a number? How, in one house,
Should many people, under two commands,
Hold amity? 'Tis hard; almost impossible.

GONERIL
Why might not you, my lord, receive attendance
From those that she calls servants or from mine?

REGAN
Why not, my lord? If then they chanced to slack you,
We could control them. If you will come to me,--
For now I spy a danger,--I entreat you
To bring but five and twenty: to no more
Will I give place or notice.

KING LEAR
I gave you all--

REGAN
And in good time you gave it.

KING LEAR
Made you my guardians, my depositaries;
But kept a reservation to be follow'd
With such a number. What, must I come to you
With five and twenty, Regan? said you so?

REGAN
And speak't again, my lord; no more with me.

KING LEAR
Those wicked creatures yet do look well-favour'd,
When others are more wicked: not being the worst
Stands in some rank of praise.

To GONERIL

I'll go with thee:
Thy fifty yet doth double five and twenty,
And thou art twice her love.

GONERIL
Hear me, my lord;
What need you five and twenty, ten, or five,
To follow in a house where twice so many
Have a command to tend you?

REGAN
What need one?

In his book, Gard calls this "comic/awful logic"--we laugh at it at the same time that we are horrified at it. In Fanny's case, he calls it "lethal rationality" and I think that fits.

He writes:

The real charm of this slightly insane logic is that there is nothing really wicked about it. It resembles that of Lear's daughters in having its own internal consistency and reductive drive;