a village where the book made a huge deal about the fact that everyone looks the same due to isolation and inbreeding.

Two particular families were called out as marrying into one another often, not the entire village. In fact I feel like Robert Jordan generally does the opposite of what you describe here - he goes out of his way to uniquely describe a ton of Emond's Field residents, and gave them all pretty varied appearances. Heck, two of the main characters, Mat and Perrin, are different as can be in appearance, aside from the fact that they both have dark hair.

Anyways, either way, even if what you're describing is accurate, it's still not a good excuse to get upset that they casted dark skinned actresses for Egwene and Nynaeve.