Believe it or not I think there are laws in some places against that. I know there are for adopted/foster children but I'm pretty sure I have read on other boards (maybe even here

) where there are places where genetic siblings of different sexes, after a certain age, can't share rooms.
I may be totally wrong, but I'm pretty sure I'm remembering right.
Personally, I wouldn't do it. I have one girl and two boys, my two boys share a room now and even that is hard for them. They are 3 years apart and the older one is starting to really want his own time away from his little brother.
My dd is the oldest, and through the years has wanted her privacy more and more, there is no way she would ever share a room with either of her brothers.
If it were me and I loved the house I would see if there was a way to modify rooms, finish a basement or attic or add an addition., If none of that was an option I wouldn't buy it.