Don't know, but I'm going to hazard a guess and say the the cops tested the finger's DNA and found a bunch of alleles that matched those of the woman. She must not watch CSI to know that family member's DNA have a lot of commonality.
I saw a brief interview of this lady who didn't sound like the sharpest knife in the drawer. That's when I got to thinking that she probably planted the finger, hoping to score some big lawsuit bucks. What an idiot. DNA and a close (relative!) connection to the "victim" would be the first thing authorities would look into.
Also if the aunt was related through the maternal line, the mitochondrial DNA should be identical. The mitochondrial DNA is located in the mitochondria not the nucleus, and humans gets all their mitochondria from the egg cell and none from the sperm that formed the zygote are the original source of all the body's cells.