The bystander older sister lady is always busy. While observing my younger sister who is always complaining that it's unfair, unfair, somehow I ended up getting engaged to the Crown Prince? What is going on here? My younger sister keeps saying "Big sister, that's cheating!" and it's so annoying!

I am the older sister of a selfish younger sister who is always cunning and takes my things away as she pleases. Since we are bound by blood, I was exhausted thinking I would have to put up with this for the rest of my life. Even our parents sided with my selfish sister. I decided to observe this and live as a bystander. After I entered the magic academy and was chosen as the fiancée of the crown prince due to my high magical power, the cunning attacks began again, and I was rejected from the engagement because they said they didn't need an older sister with such a hopeless younger sister. Because of this, I fell into despair thinking my life was ruined... but what was the method by which I, such a person, obtained true happiness?