Caddy Ever After by Hilary McKay

Indigo Casson has turned into Cupid with a twist. Rose just needs to keep a promise, but when nobody seems to understand how important it is-she might not manage. Saffy, well she doesn't think she can ever face Sarah again. How could you, when your best friend needed you and you ran away? All because of a stupid balloon. Caddy thinks she's found 'the one'. So she's getting married...but not to Michael. Leaving Rose heartbroken, she needs to find out how to stop them before it's too late.

I love this book. It's written like a journal, so you get to see each child's point of view: Rose, Indigo, Saffron, Caddy, then back to Rose. All of their lives have become so different, but they all intertwine and connect, joining up like the family they are. I love the relationship Caddy and Rose have, because I can relate to how Rose feels about Caddy so much-an sister much older than you, who rushes around going through life that makes you feel left behind. Then they come back and everything's alright again.
i love this series so much, you have to try it!