The Way to Keep an Older Brother of the Heroine Safe

I accidentally took possession of someone in a 19+ reverse harem novel. The problem is that I became Roxana Agriche, the older sister of the sub-villain. My damn father kidnapped the heroine's brother. Now, is the only thing left to meet a terrible end from the vengeance of the heroine? But what if I can avoid that horrible development? “I'm also interested in this toy.” "I'll protect you until you can get out of here safely." The heroine's brother, Cassis Pedalian, will definitely be able to pay me back later. (Source: MU)