Idea: Daughter of a noble, her older brother will inherit the estate, she awakens some power that (due to tradition or so) indicates she should inherit, but she's clearly not suited to it, so her parents and brother decide to marry the two to resolve it. He doesn't touch her or anything, but she's now stuck married to her brother.
I bet Zettour (nprofile…3qrm) has read a manga like this.
