Maybe a Fox by Kathi Appelt and Alison McGhee

Sylvie is Julies older sister (by one  year) and has kept the memory of their alive for Jules.  Together the girls throw wish rocks into the river behind their Vermont home despite it is one their father's "do not" rules.  Sylvie always has the same wish- "to run faster". She runs everywhere and can only slow herself down by holding onto objects (or Jules).  One morning before school, Sylvie runs through the snow to the Slip and never returns.  A root hidden by the freshly fallen snow caused her to trip and fall into the river and drown.
Jules and her father suffer in their grief.
Meanwhile, in the woods behind the yellow home, are wild creatures with destinies.  The bear, ransacking Mr. Archer's farm and neighbors' garbage plays a vital role, unbeknownst to him.  All he cares about his stomach.  The catamount, a large cat, silently follows a war-torn veteran who is mourning his best friend, also a war veteran.  Then, Senna, a vixen, knows she was born a spirit animal who must run, run faster and keep Jules safe.
All the worlds collide for a breath-taking ending yet a sad one.
Points to discuss/warnings: death/grief/anger, spirit animals, Sylvie and Jules wonder what happens after you die and discuss how "maybe turn into wind" maybe you turn into a fox and run, run and run".  Spoiler Alert- Senna is really Sylvie's spirit who saves Jules from falling into the Slip and drowning too.
I really liked the book and thought it was well-written.  It was just so sad!!
*Borrowed from the library.