The Super Spies and the Cat Lady Killer by Lisa Orchard
Book description (from Goodreads) -
This book opens in a small town in Michigan where fifteen-year-old Sarah Cole is stuck spending the summer at her Aunt and Uncle’s with her sister, Lacey. She’s not happy with the situation until she befriends a girl named Jackie. The three girls stumble upon the ruthless murder of a reclusive neighborhood woman.