So, there's that guy who has a 'perfect' little sister who's adored by everybody at school. I am not sure, but I think he was hated by her or smth. Then, one day, he wakes up and he's a she and nobody finds it weird. Her sister is now really spoiled and loves her. Other things I remember: his sister was blonde and there was another girl who had silver hair and she also had a sister but instead of being a reliable oneechan like he was (that's why I hate describing gb stories) she was kinda dissapointing
https://www.novelupdates.com/series/when-i-became-a-girl-an-unexpected-love-quarrel-occurred/ I expect you're looking for this.
https://mangadex.org/title/22615/onii-chan-is-done-for here, recommendation from me on the topic :eyes: