Thanks for the fascinating information about your own family history.
I don't think the "Aztec princess" story is correct, though.
The origin of
Xiomara doesn't seem to be definitely known, but the most common expert guess is that it's a form of Guiomara, the feminine of
Guiomar, a name used in medieval Spain.
Another possibility is that it is a form of Chiomara, the name of a Celtic woman in ancient Galatia (modern central Turkey) whose story is perhaps the origin of the "Aztec princess" tale:
http://www.infoplease.com/ipa/A0781453.html