Some states have banned familial searching. I took a genomics class and the guest speaker was a forensic scientist, he explained the pros and cons. Clearly getting these serial criminals off the street has some benifit.

Its been done before. One of the first was the "Grim Sleeper" case with an arrest in 2010. In that case they used the police database though (california has dna of all felons.)

https://en.wikipedia.org/wiki/Grim_Sleeper "Police had found no exact match between DNA found at the crime scenes and any of the profiles in California's DNA profile database, so they searched the database for stored profiles that demonstrated sufficient similarity to allow police to infer a familial relationship. They found similar DNA belonging to Franklin's son, Christopher, who had been convicted of a felony weapons charge. According to Los Angeles District Attorney Steve Cooley, detectives then used a piece of discarded pizza with Franklin's DNA to make the link. One Los Angeles undercover police officer pretended to be a waiter at a restaurant where the suspect ate. He collected dishes, silverware, glasses, and pizza crusts to obtain DNA.[20] The identification was used to arrest Franklin after his DNA was obtained and deemed a match.[21] Saliva found on the victims established a DNA match linking Franklin to the deaths"

I can't imagine it WON'T be challenged.