Autumn of Elves — Joanne Vruno

Panic fills Aly and her grandmother when Aly's ten-year-old sister, Emily, sees a troll. The shock of Emily also having magical powers gives way as the sisters begin to share a world of intrigue. Aly becomes Emily's instructor and protector while they interact with many new creatures―some of which are fascinating and some terrifying. Aly finds her guardianship responsibilities increasing as the autumn wears on. Can she keep her sister and all the light elves safe from the dangers that await?