Northstar and Aurora are two mutant twins from the Great White North, Canada. These French-Canadian twins don't work as closely together as they used to, but they were once quite inseparable. While Northstar currently resides on the X-Men roster, his sister Aurora is more commonly associated with the Canadian super-team, Alpha Flight.
The reason these two twins used to work so closely, is due to a former limitation in their powers. At one time, neither of the mutant siblings' powers would activate without the other. Without each other in close proximity, their powers wouldn't respond, making teamwork between the pair a necessity.