(since back then males were predominately male)?
I'm pretty sure they still are!
I always figured the Aussie "sister" thing was partly a throw-back to the days when a great many nurses were women of religious vows, and also a throw-back to the days when aligning nurses with religious women appeared to be a neccessary way to make them seem more respectable, because of course to respectable woman would choose to do work that would put her in such immodest circumstances unless she was responding to a sort of religious calling, and was completely asexual.