I was a teenager when I read this series but I remember liking them a great deal.
Caroline discovers she has a long lost cousin exactly her age who lives in Iowa. Chrissy decides to move to the city and live with Caroline's family for a year. Caroline instantly regrets this move because Chrissy soon takes over Caroline's life: her bedroom, her friends and even tries stealing her boyfriend, or so Caroline thinks.
I don't know how many books were in the series. I found these in a bag in my parent's attic. I only have Books 1, 3, 4, 5, 8, 9. I imagine at some time I had them all so I don't know where the rest are. I plan on rereading these before giving them to a used book store.