I can explain why Warren's mother might have made the claim of indigenous- particularly Cherokee- ancestry.

In the 19th and early 20th centuries, there was actually a fashion, in parts of what had been "the West"-in particular Oklahoma where Warren's family was from, though it existed in other states- of white families claiming Cherokee heritage they didn't actually possess, out of the belief that such heritage was "desirable" in a way that other Native American heritage was not.

Still goes without saying that she never deserved to get put through the mill over this, since she never used it for personal gain.