Mistletoe. Poison Garden, #5

Eli and Dan go home for the holidays and run afoul of a winter deity who's been holding a grudge against Dan's family. That's going to put a damper on their Christmas spirit. Eliza "Eli" Moore has embraced her quiet life with Dan, her husband and partner in their detective agency, Nine Lives Investigations. When Dan took Eli to New York so they could spend the holidays with his family, she expected a week of home-cooked meals and ugly Christmas sweaters.
Instead, she discovered a spirit trapped in Dan's family home, a seer brewing up batches of poison in the basement, and that Dan's family had been cursed by a batch of enchanted mistletoe. As Eli and Dan search for a way to end the curse, they visit the farm where the mistletoe originated. Once there, they find the farm steeped in magic, and a proprietor that seems a little too interested in Dan's family.
When Eli learns the proprietor is actually a demon that froze his daughter to death, she vows to stop him, even if it means she gets frozen, too. And Eli hates being cold.