Japanese title: I REALLY DON'T WANT TO HEAR THAT I BECAME MY YOUNGER SISTER!
Special thanks to Super Shanko for technical help.
THIS IS THE CLIMAX FOR GENDER BENDER DAY.
Web version: https://ncode.syosetu.com/n9140dr/2/
Prologue: a guy has one sister. One day he wakes up as a silver haired, red eyed girl, and his familiar sister says "Did you mix up your bed with Oniichan again? You're too old for this." She calls him her twin sister and goes to school. As he looks in the mirror, a guy who looks just like him gets up, with his memories and asks who she is.
Together the two check the photo album, and conclude they both must have come from parallel worlds.