I have a similar situation. Also 1/8th, from my Great Grandmother. My Grandpa was also a member of the tribe and I spent the first part of my life living near the reservation and used to visit. My mom could have been a member as well, but she was bullied by the Native American kids and that turned her away from that side of her heritage. I am not sure if I should check the box, but I am definitely considering writing a diversity statement. Perhaps this is a question for the individual schools.