This is cliché as fuck. A cold-hearted, black-haired northern duke, an OGFL who is a white lotus, a too good for this world MC. Also, ignoring the reality of nobility succession issues again.
I was sooo fucking right. Her younger sister stole the power of the family head (haven't heard that one before /s). The Clown Prince (OGML) gets interested in her "because she's no longer into me". Oh, and the Duke's family are the first ones to love her, since they hope for a successor. Is there any trope this story doesn't milk?