Your family may very well have native American in them. Your parents each contribute 50% to you, but the 50% is different for each child. Note, a sister isn't an exact copy of you, so she gets a different 50%. Also, because your brother has a y chromosome, his fifty percent will be even more different. Have your brother take the test because his test will also show more from your father's side of the family. Have your parents take the test. They may have native blood that simply didn't get passed onto you.