The murderer has to leave the DNA sample somewhere in the scene or on the victim. That may be semen, a tossed cigarette butt, a tossed drink can or bottle, etc.
Simply having a relative submit their DNA will not result in a match without DNA from the crime to match it against.
Besides, I don’t care if they match my DNA against a relative, however close, who has murdered someone. Son, daughter, sibling, cousin; if they murdered someone then they need to be brought to justice, period.
The only time it “might” be a problem is if you have an identical twin who committed the crime and even then the DNA might well have identifiable differences but that would be a statistically low probability.
The murderer has to leave the DNA sample somewhere in the scene or on the victim.
No they don't.
Crooked or incompetent cops, prosecutors and crime labs can make it happen through selective analysis, cross contamination, poor record keeping...you name it.