Splinters of scarlet / Emily Bain Murphy.

Enhanced descriptions from Syndetics:

"Part wish-fulfillment fantasy...part gritty whodunit,"* Splinters of Scarlet is a spellbinding and atmospheric historical fantasy set in nineteenth-century Denmark, where secrets can kill and magic is a deadly gift, for fans of The Gilded Wolves and Pride and Premeditation. (*Kirkus)

"Emily Bain Murphy weaves an exquisite tale of mystery, enchantment and valor. I loved this spellbinding book!" --Rebecca Ross, author of The Queen's Rising

For Marit Olsen, magic is all about strategy: it flows freely through her blood, but every use leaves behind a deadly, ice-like build-up within her veins called the Firn. Marit knows how dangerous it is to let too much Firn build up--after all, it killed her sister--and she has vowed never to use her thread magic. But when Eve, a fellow orphan whom Marit views like a little sister, is adopted by the wealthy Helene Vestergaard, Marit will do anything to stay by Eve's side. She decides to risk the Firn and uses magic to secure a job as a seamstress in the Vestergaard household.

But Marit has a second, hidden agenda: her father died while working in the Vestergaards' jewel mines--and it might not have been an accident. The closer Marit gets to the truth about the Vestergaard family, the more she realizes she and everyone she's come to love are in danger. When she finds herself in the middle of a treacherous deception that goes all the way up to the king of Denmark, magic may be the only thing that can save her--if it doesn't kill her first.