The scenario would be this: teenage girl, newly orphaned, certain things are coming up to make her suspect (correctly) that her parents weren't really her parents. So she steals her older sister's toothbrush, orders a sibling DNA test off the internet, and that's where I don't know what would happen.
The sister would actually be her mother: got pregnant when she was 14, the parents covered it all up and raised the granddaughter as their own. Everything I found says that parents and full siblings are both 50% matches, and mitochondrial DNA would be the same either way. So would the test give a false positive? Or would there be something that would let the lab identify the real relationship?
ETA: I know 50% is only an average for non-identical siblings. I'm wondering what results she would get with only her sister as a comparison.