YA southern gothic book about sisters and there's a swamp

[Glennon]

Hi all,
I'm trying to remember this book I read around 2008-2011. It is a YA book, I believe. It had dual perspectives between to sisters with an age gap of maybe 3 years. It had a southern gothic feel to it, and I believe witchcraft was involved. It also had a creepy swamp. I remember one scene was from the older sisters perspective and involved her bad boy boyfriend going down on her in the swamp.
I can't for the life of me remember what this book is.
Thanks for any help you can give.