Reikoku na Koushaku-sama wa Nanashi no Okazari Tsuma ga Okiniiri: Akujo na Ane no Migawari de Kekkon Shita Hazu ga, Kigatsuku to Dekiai Sarete Imashita

A young lady born as a twin was hidden from the world and never even given a name—simply called "Nameless" because twins were considered unlucky. Raised like a servant, she secretly found small joys, inspired by her nursemaid's words to become a "happiness master." One day, a marriage proposal meant for her older sister is forced upon her. She must marry the infamous cold-hearted duke, Silvance, in her sister's place. Rushed through lessons to impersonate her sister, she arrives at the ducal estate expecting the worst. But the first words she hears from him are unexpected: "If you can maintain a platonic marriage for two years, I will grant you a divorce." Embracing her new life as an ornamental wife, she finds it far more comfortable than her childhood home. Grateful, she tries to help soothe her husband's work fatigue—and soon, he begins visiting her daily, his cold smile warming into something unexpectedly tender.

https://www.cmoa.jp/title/351721/