In chapter 10, Anne has been observing the three of them (FW, HM, LM) and reaches a conclusion:

Other opportunities of making her observations could not fail to occur. Anne had soon been in company with all the four together often enough to have an opinion, though too wise to acknowledge as much at home, where she knew it would have satisfied neither husband nor wife; for while she considered Louisa to be rather the favourite, she could not but think, as far as she might dare to judge from memory and experience, that Captain Wentworth was not in love with either. Chapter 10

In the next sentence she notes that, while they were not necessarily in love with him, it looked like that was where it would lead:

They were more in love with him; yet there it was not love. It was a little fever of admiration; but it might, probably must, end in love with some. Chapter 10

Later in the chapter, after Henrietta made her peace with Mr. Hayter:

Every thing now marked out Louisa for Captain Wentworth: nothing could be plainer; and where many divisions were necessary, or even where they were not, they walked side by side nearly as much as the other two. In a long strip of meadow land, where there was ample space for all, they were thus divided, forming three distinct parties; Chapter 10