I'm not really the expert here, but I'll share what I know. You get half your active genome from your father, the other half from your mother. So do all your non-identical-twin siblings. You and your sibs are all mixed halves of your parents, but you're all unique mixes. You get your mother's eye color and your father's hair. Your sister gets your father's eyes, your mother's nose, and the hair color your mother carries recessively from her grandmother. The result is that you and your siblings may not have the same blood type, may not be able to exchange skin or organ grafts.
So it's possible that a child could have specific genes that can't come from her aunt, if anyone wants to sequence the entire genome of both individuals. But I bet the typical DNA test doesn't do that and won't be sensitive to that level of difference. I suspect a lot depends upon just what tests are done and how sensitive they are.