Isn't that quite similar to help solving cold cases as example:

https://en.wikipedia.org/wiki/Joseph_James_DeAngelo

On April 24, 2018, authorities charged 72-year-old DeAngelo with eight counts of first-degree murder, based upon DNA evidence; investigators had identified members of DeAngelo's family through forensic genetic genealogy.

Lets say there's some rare genetic disorder, only a few hundreds of a percent of the population has it. If someone knows that your mother or father has it, you now don't have a few hundreds of a percent chance of having it. Depending on the disorder you having it might just be a cointoss.