The first lines of Madeleine Robins' three Sarah Tolerance books are as follows:
Point of Honour
It is a truth universally acknowledged that a Fallen Woman of good family must, soon or late, descend to whoredom.
It is one thing, and a quite considerable thing, to be a lady.
The Sleeping Partner
No one who had seen Miss Sarah Brereton as a child would have taken her for a heroine.
The first and third lines are obvious references to the first sentences of Pride and Prejudice and Northanger Abbey respectively. The second one doesn't come from the first line of any other Austen book; is it a reference to something else? I've been googling fragments of it, but I haven't come up with anything plausible...