Help me find a book and/or fanfic where the trope is about the second princess who’s in the shadow of her older sister

My friend wants a book where it's basically like the spanish royal family (her words not mine), and it's about the struggles with being in her sister's shadow and pushed to the side and there's angst and good plot line, she doesn't care what genre of the book is either.

Is there anything out there like this?