[ptaipale]

I don't see it as very surprising that your mother and her sister look like they belong to different races. That's how genetics works in case of individuals; siblings can get different sets of dominant and recessive alleles from their parents.

In terms of population, things then average out. There are indeed parts of the world where things are very blurred because of extensive human interaction across populations. If every place in word were like that, then there indeed wouldn't be what we call races. But most of the world is not quite like that, and particularly, in the course of history it hasn't been.