Dueling Magics by Stephanie Burgis is currently free to download (in various formats) at Smashwords.
Kat Stephenson will do almost anything to escape a tedious shopping trip with her stepmama and older sisters...but when she enters a magical duel of wits with her sister Angeline, no one can predict the results!
"Dueling Magics" is a short story set a few months after the events of Kat, Incorrigible and a few weeks before the beginning of Renegade Magic (Kat, Incorrigible Book 2).
1. US = Kat, Incorrigible, UK = A Most Improper Magick
2. US = Renegade Magic, UK = A Tangle of Magicks
3. US = Stolen Magic, UK = A Reckless Magick
(All currently 99p on UK Kindle.)