What is ethically problematic about using DNA to find relatives of an unknown offender?

The search is widened to "similar" DNA so that relatives of the offender can be found. It uses databases built for other purposes, pulls innocent family members into an investigation, and may get around the right to refuse to testify against relatives.

Relatives share part of their DNA, so a partial match in a database points to a family member of the offender. That family member is then used to get to the offender. Three problems:

• Purpose limitation: the DNA was stored for a different reason (zweckfremde Datenbanken), for example in an earlier case or a genealogy service, and is now used to investigate someone else.
• Innocent people as investigative tools: the relative is not suspected of anything, yet their DNA leads the police to their family.
• Circumventing the right to refuse testimony (Zeugnisverweigerungsrecht): in criminal proceedings, close relatives do not have to testify against each other. A familial DNA search makes a person's body "testify" against their relative without their consent, which the law otherwise lets them refuse.

Go deeper:

• Wikipedia: DNA profiling, familial DNA searching — near-matches as a route to an offender's relatives, and the privacy debate.
• swissinfo: Switzerland adopts DNA phenotyping — the Swiss law change, including searches for relatives, and its critics.