I can understand things coming out in the wrong way at that age - I never had to share a room with him, but there's 12.5 years between me and my brother, and I was an only child until then.
I adored him, I'd begged for a sibling for years, but he was also the worst thing that had ever happened to my teenage self. When circumstances meant we moved to a smaller house, and then my other brother came along (also very much adored, then and now), and I had to move from my small single to a room barely big enough for my bed, a bookcase and a chest of drawers, yeah, I did resent it. I did have a (teeny tiny) space to get away, but the rest of the house was occupied by them.
I empathize with her. Once they are sharing full time, if, as PPs have suggested, there's a way of giving her somewhere to hole away - even a decent garden shed hooked up to the electrics with a desk in it for studying in - that would be what I'd aim to do.
Anecdotally, on the "it was different in the past", my grandmother (5th of 6) shared a bed with her youngest sister until she moved out. Not in the very distant past - this was the 40s and 50s. Times really do change.