Jeanne Birdsall, The Penderwicks: A Summer Tale of Four Sisters, Two Rabbits, and a Very Interesting Boy. Yearling Books, 2007.
The Penderwicks is about a family of four girls, (from oldest) Rosalind, Skye, Jane, and Elizabeth (Batty). Their Dad is a renowned botany professor. They decide to take a special vacation to a little summer home right next to a manor, instead of their usual cottage in Maine. While they're staying at the vacation home, they get caught up in family business of the people who live in the manor. They also make friends with the neighbor's son, Jeffrey. As they get to know the lady better, they grow to like her less and less, and when they learn Jeffrey has to go to military school, they can't bear it any longer, and work together to stop Jeffrey from going to military school.
If I went any longer I would give away the ending, and trust me, you don't want that to happen.
I loved this book and there are two more in the series that are just as awesome as this one.
If you like thrills, action, and suspense throughout the book, this is the book for you!