After you get into the first hidden place --
What hidden places?
-- you'll find a family tree that will help you keep track. In the meantime, though, here's what you need to know.
Jessalyn is the girl who's missing. Colton is her fiance.
Clara is Jessalyn's mother. Her mother Rosalie died when Clara was young, and Clara doesn't know who her father is. How sad!
Wade is Clara's cousin (which makes him Jessalyn's cousin once removed). He and Savannah (the woman who asked Nancy to investigate this mystery) dated for a while.
Charlotte and Harper were sisters; Clara is a cousin who came to live with them when her mother died.
Still unclear? Well, it's not that important just yet. After you get inside the first hidden place --
-- you'll find a copy of the family tree, and the relationships will be easier to untangle.