She's the One Who Gets in Fights

Sulphur, the third of seven sisters, is glad the older two have been slow to wed. It’s given her the freedom to train as a fighter, in hopes of fulfilling her lifelong dream of joining Ilari’s army. Then, within a matter of days, both sisters announce plans and now Sulphur is expected to find a man to marry.
Is it Sulphur’s good fortune her homeland is gripped by fear of a pending Mongol invasion? And the army is going door to door encouraging recruits? Sulphur thinks it is. But once she’s forced to kill in a small skirmish, she’s ready to rethink her career decision.
Too bad it’s too late. The invasion is coming, and Ilari needs every good soldier it has.
Once Sulphur learns Ilari’s army has made the strategic decision to not defend certain parts of the realm, including the one where her family lives, she has to re-evaluate her loyalty. Is it with the military she’s always admired? Or is it with her sisters, who are hatching a plan to defend their homeland with magic?
Everywhere she turns, someone is counting on her to fight for what’s right. But what is?