The Brightest Fell by Seanan McGuire

For once, everything in October “Toby” Daye’s life seems to be going right. There have been no murders or declarations of war for her to deal with, and apart from the looming specter of her Fetch planning her bachelorette party, she’s had no real problems for days. Maybe things are getting better.

Maybe not.

Because suddenly Toby’s mother, Amandine the Liar, appears on her doorstep and demands that Toby find her missing sister, August. But August has been missing for over a hundred years and there are no leads to follow. And Toby really doesn’t owe her mother any favors.

Then Amandine starts taking hostages, and refusal ceases to be an option.