This week I read a novel where a lot of interesting and surprising things happened, plotwise. It was a very active story and you couldn't predict where things were going as the events careened from chapter to chapter, carrying the characters along with them. Which is exactly the problem with this book.
Part of what made the plot twists possible was the author's annoying habit of changing the characters however she needed to in order to make them into people who'd fit the new plot threads. After about 140 pages of this I realized that I didn't know who any of these people were, because they weren't anybody at all. None of the characters were centered. This caused a great deal of emotional disconnect for me, because since the characters never became well-defined, I stopped trying to care what happened to them, because they were more like a faceless mob than a half dozen real people.
It is possible that, because this is a first-person narrative and the protagonist is blind to a lot of the character traits of the people around her, all of this shifting of identities is meant to be a sort of voyage of discovery for the narrator and the reader. Gradually we see beyond the prejudices of the narrator and find out who all her friends and relatives truly are. But on the page, it doesn't work that way, and you can see how the character changes exist merely to allow the plot to move forward in its clunky, episodic manner. Chapter after chapter, the narrator is saying things like "I could see that my daughter was no longer shy" or "I never knew my husband was so depressed" and the problem is that, if you're going for irony here, the reader has to have seen it when the narrator hasn't. No, the daughter was shy when the story needed her shyness, and she was outgoing when the story required an outgoing daughter. It was all very improbable.
So my advice to you is this: as your characters travel along within your narrative, take the time to ask yourself if each character would, given who you say they are, behave the way you make them behave in each circumstance. You should probably do a read-through of the MS with nothing else in mind than to make sure that the characters are consistent all the way through. My characters have a knack for being a bit differently imagined at the end of the first draft than when I started, and I have to spend time making the characterizations match up at each end of the story.