Giving it to police, and then police using it to convict Person B? Is there a due process issue? Say hypothetically you had a nuclear family with only two children, Leia and Luke. Leia really wants to figure out her birth line so she submits her DNA. Meanwhile, Luke has committed a nonviolent felony a few year ago and left behind some blood in a snatch and grab. Other than the blood, there is no physical evidence that would link Luke and no circumstantial evidence. The police compare Leia's DNA and it matches closely as a sibling of the theft. The police arrest Luke who is subsequently convicted.

Thoughts?