Daiichi Oujo Lucia no Kikan to Kareinaru Kaishingeki Umi no Shugo Kishi to no Deai

Princess Lucia, heartbroken after the death of her fiancé, the crown prince of another country, returns to the Kingdom of Fortnato. There, she becomes embroiled in a power struggle for the throne among her four younger sisters, endures harassment, and is eventually driven out of the palace! However, encouraged by Felix, the duke's son, who is drawn to her nobility, Lucia displays her "royal qualities" in her new home!

She emerges as the leading candidate for queen—!?

(Source: Kadokawa, translated)