Before reporting the supposed crime, I'd ask myself a lot of questions:
First, how strong a piece of evidence is an admission of guilt? Increasingly, psychologists and legal scholars are discovering how remarkably common "false confessions" are: http://courses2.cit.cornell.edu/sociallaw/student_projects/FalseConfessi...
Is there any reason your relative might admit to this crime aside from having actually committed it?
Second, you say that there is not enough information to lead to an indictment or trial. But of course that determination isn't yours to make, and it may be that once your relative is under suspicion, an investigation will yield more information about the crime, information that could lead to your relative's being indicted. So do you really know what you claim to know, i.e., that there's not enough evidence to indict (as opposed to your not having access to sufficient evidence to indict)?
Third, you say that there aren't any "significant real world consequences" for the relative if you report this. But again, that's not clear and not solely up to you to determine. Do you believe in good conscience that your relative ought to suffer the punishment associated with murder (a lengthy prison term, or in some places, execution) if the relative in fact committed the crime, and if so, why? (Here's a quick primer on how to think about the ethical justification of criminal punishment: http://plato.stanford.edu/entries/punishment/)
Lastly, I'd point out that in most legal systems, you would be required to "incriminate" a close family member unless the individual is your spouse. So are you willing to play such a role in the potential prosecution of this family member?