I agree that she is regretting her engagement and setting her sights on Capt. Tilney. Luckily, she's projecting again, so she gives herself away with several comments.

First, consider her description of Mr. Tilney at the beginning of Chapter 16: "And then the brother he, who had appeared so attached to you! Good heavens! Well, some people's feelings are incomprehensible...How contemptible! Of all things in the world inconstancy is my aversion. Let me entreat you never to think of him again, my dear Catherine; indeed he is unworthy of you...Such fickleness!" Even though Capt. Tilney has not yet arrived, the groundwork is set for Isabella's own fickleness, and we are all too aware that she is unworthy of James Morland.

Then, consider her criticism of Catherine in Chapter 18: "Nay, my sweetest Catherine, this is being quite absurd! Modesty, and all that, is very well in its way, but really a little common honesty is sometimes quite as becoming. I have no idea of being so overstrained! It is fishing for compliments." Kathleen (elder) observed in a previous post Isabella's fishing for compliments and pretending to be modest. And there is little common honesty about her own behavior at this point. Imagine her telling Catherine, "Yes, yes" (with a blush), "there are more ways than one of our being sisters." Hmmm.

After accusing Catherine of being engaged to Mr. Thorpe, Isabella absolves Catherine of any wrongdoing: "A little harmless flirtation or so will occur, and one is often drawn on to give more encouragement than one wishes to stand by. But you may be assured that I am the last person in the world to judge you severely. All those things should be allowed for in youth and high spirits. What one means one day, you know, one may not mean the next. Circumstances change, opinions alter." It is not difficult to guess that Isabella is revealing her own justification for her behavior to James Morland.

Finally, in the worst case of projection, Isabella gives Catherine the advice she wishes that Catherine would give her: "My dearest Catherine," continued [Isabella] without at all listening to her, "I would not for all the world be the means of hurrying you into an engagement before you knew what you were about. I do not think anything would justify me in wishing you to sacrifice all your happiness merely to oblige my brother, because he is my brother, and who perhaps after all, you know, might be just as happy without you, for people seldom know what they would be at, young men especially, they are so amazingly changeable and inconstant. What I say is, why should a brother's happiness be dearer to me than a friend's? You know I carry my notions of friendship pretty high. But, above all things, my dear Catherine, do not be in a hurry. Take my word for it, that if you are in too great a hurry, you will certainly live to repent it."