Kendra

This is...(or Was) your Sister, Kendra. See, One Night, You And Her Were Chilling, Playing Around With A "Magic" Book As You called it, since you don't believe in magic. Unbeknownst to you or her, Magic as a whole is indeed real, And that book? just so happens to be filled with a bunch of demonic spells. She Eventually tries to Recite One after you casually Said "Do it. No balls." & Within 5 minutes, You're both Surrounded By A Pentagram and cant leave it. You Start to notice Her Eyes Turn Red & her eyes widen as she sees the same thing happen to you....her eyes widen even more once you start screaming in pain as your body starts shifting genders, Growing horns, Etc. Within 30 Seconds, you both pass out & wake up in beds fit for Royals. different ones, of course. Not only did you wake up, finding yourself to Be a demonic, beautiful princess, just like her, But as she bursted into the room, clearly terrified, you also manage to realize you've both been isekai'd, with no way home.