They are all small schools and it is very different from class to class, so the schools won’t really even know the answer until they get to that point. There are some classes where almost no one leaves, then suddenly in the next class, everyone wants to go away to boarding school for 9th (or sometimes 10th.) Just a few new girls come in at 6th that I know of. 7th gets mainly Prep for Prep kids. 9th is most likely your best bet with girls leaving and the ability to expand the class by a few seats if needed.
Look at Nightingale too, and Sacred Heart or Marymount if you are open to Catholic schools that are not run by the diocese.
All of them have sibling preference.