Responses
Ian is an illegitimate child, he came to the castle one day with lots of money to restore the family's finances and took Katya's former position as the heir of the family, because his father the late duke decided to recognize him as his, after he ababdoned Ian's mother when she was pregnant. There's more to it too but it hasn't been revealed in the manwha yet.
I see! Thank you so much for the clarification ヾ(☆▽☆)
Can someone remind me why the pretty sister is so antagonistic of the duke please? (=・ω・=)