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

Following the death of her fiancé, the first princess, Lucia, returns to her home country. Upon her return, she finds herself subjected to daily harassment by her younger sisters, who are vying for the throne. However, with the support of Felix, the duke's son, her "royal qualities" begin to blossom...!? (Source: Kadokawa, translated)