Ruby Reads ''Fireflies' by Carolyn Watson- Dubisch.

'Sisters, Caitland and Cassidy take the ferry one night. Under the light of the full moon, the fog rolls in and tiny, glowing lights surround them on the upper deck of the boat. "Fireflies!" whispers Caitland in trembling excitement.
"Get Them!"
Then they begin to make the greatest mistake of their lives."