When I Became a Girl, an Unexpected Love Quarrel Occurred!

I woke up to my twin younger sister lying beside me. She was a famous beauty in our school, unlike the ordinary me. Though the true personality of this perfect girl was somebody who was especially selfish, although she only ever showed it to me. Although we never got along it was really weird to see her be anywhere close to me. She then opened her eyes, smiled at me, and said.
“Good morning, Onee-chan.”
Eh?
I was supposed to be an Onii-chan and somehow became an Onee-chan.

I was a plain older brother and now I became a plain older sister. I would have preferred if I somehow got transformed into a woman, I would at least be beautiful, but I guess life isn’t that convenient.
Anyhow, my sister is considered one of the two most beautiful girls in school, and now she is always fighting with the other most beautiful girl. The reason? It’s over me. I don’t know why but for some reason I am in the middle of this. So hold on me, I won’t be defeated. But if anybody could help, please save me.