They've been giving it to cops for a while. They'll use the extended family to pin point their unknown person, so like if you have some third cousins that have gotten in, they'll possibly be able to find you. If a parent has their information in, or both, well your family for a few generations will be fucked.
im not a geneticist but knowing how much cops suck & misuse dna 'evidence', i can't really imagine how knowing a suspect could be in the hundreds of an extended family actually be... they must be using it mostly to get around having to procure a warrant to test people's dna
sorta. it lets you narrow down existing suspect pools, or open up new pools that you can then narrow down by more traditional means. but probably all of the above and worse are happening cause its cops