Which is fine, I suppose, but your relatives don't have to do delete theirs, and the similarity of their DNA to your DNA means that someone could back out, to a certain level of confidence, whether you might be a match based on DNA contributed by them, with the familial relationship obtained and substantiated from other sources (vital statistics, etc.).

This is not new -- I believe there have been at least a couple of arrests made as the result of a suspect's relatives contributing DNA in lieu of actually obtaining one from the suspect directly -- and provided there's consent from the relatives, I can't see much grounds from restraining someone from allowing their own DNA to be used to build a case against someone else. It becomes a bit murkier if you start compelling people to contribute DNA to build a case against a family member, but even there I can see parallels to other types of evidence and it's not a clear slam-dunk to me that it ought to be barred unless the familial relationship is one where testimony would normally be covered by marital confidence or other privilege.