Wakeari Hakushaku-sama to Keiyaku Kekkon Shitara, Musume (Roku-sai) no Keiyaku Haha ni Natte Shimaimashita.

Maria Martini, the daughter of a baroness, is contractually married to Count Ricardo Ricœur for one year only. Ricardo's wife has been unfaithful to him and left him, and he has been repeatedly honey-trapped by women who want to marry him again. Maria's role was supposed to be a cover to protect him, but during the course of their life together, Ricardo's behavior became strange. The first thing that comes to mind is that Ricardo's stepdaughter, Lydia, is very interested in her new mommy. A contractual marriage begins a too-happy married life. Source: MU