Not sure why this is so controversial unless the probability of DNA matching of the family member to the perp is so low that asking for a sample would be an unreasonable search. Lawyers are typically very challenged when it comes to hard sciences and so typically make very bad decisions (just look at the application of the endangered species act). The fact that California is using this makes me suspect that the science isn’t too good on this so would like to see a better explanation before I would say this is a good thing or not.
The DNA inference from parent to child or vice versa would need to be well nigh perfect to stand up in a criminal court as primary evidence. Where things might get cloudy is where the match is known to be badly imperfect but the government wants to use that as probable cause for search or seizure. On the other hand, could a poor match through a parent be viewed as the equivalent of an alibi without the suspect ever having to surrender a permanent sample of his own DNA?
Imagine being the only known male relative of the guy on file in a case like this, and the SWAT bangs in your door. Except that you have a half brother you didn’t know about - nobody knows about him including your dad. DNA will eventually clear you but in the meanwhile you’re in for some considerable unpleasantness.