Wouldn't a more apt comparison be Aryan Nations? They also want to carve out a homeland from within the United States, likewise based on ethnic identity. Heck, they want the Pacific Northwest, and the Aztlán folk want the Southwest. No overlap in their claims, and plenty of common interest. They should be working together! (I joke, and then it will probably turn out to be the case.)
Ask your neice what she thinks of the very similar demands and aspirations of white racists who want an "Aryan Nation". If she claims that they are not natives with a rightful claim to the land, you can remind her that the claim of hispanics to the North American Southwest is also dubious, as they stole those lands from North American tribes. (At this point you will have to remind her that treating all "native americans" as interchangable is a bigoted manipulative tactic of the white, European oppressors. Native Americans certainly never thought that way before their perceptions were contaminated by the so-called "ideals" of the white devils.)
You might also remind her that the Aztecs were oppressors who stole lands from native peoples right and left, establishing and maintaining their empire by military conquest, and dominance of the peasantry by small priviledged elites.