You Can't Bluff the Sharp-Eyed Sister 1 (manga)

A shrewd, young priestess-in-training is set a murder to solve in this new mystery manga from the creator of The Apothecary Diaries! By day, Chloe lives at a convent. By night, she's such a card shark, a holy knight ends up believing she has a rare, magical gift. What he doesn't know is, she just has sharp eyes and a quick wit--which she'll need when she's uprooted and dropped in the middle of a round of deadly palace intrigue! In a sleepy countryside church, Chloe plays the part of a prim and naïve novice, studying to become a priestess.

But when night falls, she sneaks out in disguise to win big money at cards! Despite her best efforts, word gets around, and one day a handsome young holy knight named Heraldo—known as a rising star in the capital—arrives to enlist her help. A murder two years ago threatens the stability of the kingdom, and Heraldo thinks this card shark in sister’s clothes can solve it using her Gift—the rare blessing that allows someone to use magic. Little does Heraldo know, there’s nothing supernatural about Chloe’s winning streak.

She’s just got a quick wit, and sharp eyes! Will that be enough to survive the den of devils that is the royal palace?