To me it’s that the evidence didn’t come from the suspect. It’s like being able to recognise me from a photo of a relative on FB - I don’t like the idea of that. I don’t know why, but I don’t.

What's the difference between this case and the fact the unabomber was caught because his brother and sister-in-law recognized his writings?

What's the difference between this case and if the cops post a photo of a suspect and someone says "looks kinda like my co-worker, maybe a close relative" and that co-worker says "oh, that's my brother, we look alike, people mistake us for each other all the time." And then the DNA matches the brother.

That the relatives gave evidence unknowingly and that the DNA service was used quite differently to what it was designed for. A terrible person was caught in a clever, but sleazy way.

The police used GEDMatch which is openly available and it says explicitly it's used for "researcher" and warns users that their genetic information is available to the public and they can't control what the public does with it.

People "unknowingly" give evidence to the police all the time. That's like complaining the police used information that someone blogged about to solve a crime.

The issue as I understand it, is with the broadness of the search, and that the data in question is personal and private. The search did not query for a particular subject. As an analogy, if the police thought the suspect had diabetes, would they be able to subpoena a hospital to list all the diabetics, then consider them all potential perpetrators?

Or, literally, fingerprints.