Sounds like to me that you may still have Native American ancestry, but it's not showing up. Perhaps if your sister take the test, her test might show a small percentage of Native American?? As explained to me in a previous post, the admixture test evaluates the traits you personally inherit....and not necessarily the measure of the ethnicities of your ancestors within a certain number of generations. So..perhaps your DNA didn't have traceable Native American traits....but your sister's might?? I am really not sure.