Let me put it this way. The government of the Métis nation has adopted criteria as to who is eligible to be registered as a citizen of the Métis nation, not who is of mixed ancestry—we have no control over that. Pretty well all indigenous peoples in this country are of mixed ancestry, but they're not Métis.
In terms of us, we do have registries, yes. The Métis Nation of Alberta has a registry, and of course you're from that province. Those who meet the criteria are registered as citizens; they have that right. If they don't meet those criteria, then they're not registered, because they're not part of, or citizens of, the historic Métis nation. They may be people who have moved—in the case of Fort McMurray—from Newfoundland, for example, or somewhere else in the east. They would not qualify.
When you say a brother and sister, I don't know the exact circumstances of that, but it seems strange to me. Either the one should not have been registered or they both should have been registered.
Basically, when we talk about the Métis nation, we're talking about a distinct people based in western Canada, although some now live in other parts of Canada, and they're entitled to be registered as citizens of the Métis nation. If you're in Australia, you're entitled to be registered. It doesn't matter where you live in the world, as long as you're a descendant of the historic Métis nation, as long as you're entitled to be a citizen. We're going on the basis of nationhood as a sociopolitical group and as a historic people.