Carrie Marie Underwood was born to Stephen and Carole Underwood in Muskogee, Oklahoma, and was raised on her parents' farm in the rural town of Checotah, Oklahoma. She has two older sisters, Shanna and Stephanie, and is a member of Muskogee Creek Nation of Oklahoma.
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If I remember, it is on the side of Carrie's Father.
I have only seen this on the Wikipedia site, not from an official Carrie site (I have not checked in some time).
The blood quantum requirement for the Creek Nation is not
1/4, it is 1/32, the last time I visited the citizenship office that is.
All North American tribes set their own blood quantum requirements, every generation the blood quantum requirements are allowed a lower percentage. Cherokee Nation of Oklahoma is very low, there is talk someone received citizenship with a mere 1/1000 degree of Cherokee Blood.