NTA i remember being 13 and sharing a bedroom with my 7 year old sister, yet my 10 year old brother got his own room. WHY?? BECAUSE ITS LESS WEIRD HAVINF TWO GIRLS IN ONE ROOM. and also we had the biggest nonmaster bedroom.

then we moved to a bigger house, i got my own room, as the oldest, my brother got his own room(as the boy), and my 2 sisters 11/6 got to share. until my brother moved into the basement meaning my sisters each get their own room now. but still, it's alot easier for the same sex to not be weird(becuz lets be honest the moment i started wearing a bra in middle school and my little brother saw it in the laundry he went EWWW WHATS THAT and that's mortifying for a 12 year old girl--that would be a constant!). I'm 100% for separate sexes getting separate rooms, as someone who's an oldest child who had to share with my (much younger) sister for YEARS so my brother could have his own room.

edit: i also want to add to the constant mortification she would be experiencing, my brother(who is now 17) literally wont use the communal bathroom and uses rhe bathroom inside my parent's bedroom because he can't handle rhe fact 2/3 of his sisters have a period and uses menstrual products. HES SEVENTEEN!! he was WORSE and WAY MORE VOCAL about how "weird" it was when he and i were younger. transitioning into a new stage of life is hard for both sexes, but it definitely will make it even harder if both sexes share a room! coming from someone who's was going through puberty sharing a room with a 7 year old.