If I remember correctly and someone can correct me if I am wrong, but I watched a tv program about the Unabomber. One thing I never knew was that he was turned in by his own brother. It turns out when the Unabomber wanted the manifesto published in the papers, his brother picked up the newspaper and could tell by the writing and the ideas expressed that it was his brother. But because he seemed to be very sure, the brother did what he thought was the right thing.
When the Unabomber, Ted Kaczynski, was brought in by investigators he was obviously very surprised that LE found him. So he asked the question "How did you find me?" and if I remember correctly, LE told him the truth. Someone correct me if I am wrong. I think at some point the family probably would have told him anyway. I think the special I saw was about family members who are from a killer's family and how they live with the last name, etc.
In the Delphi case, there is always the possibility that a family member may recognize the man from Liberty German's phone video, but I think it will be very difficult to be sure about it. Let's assume for a moment the real killer is 58 years old and lives in South Dakota. This is an example. Now they research the case before submitting a tip because they want to be sure and the only thing they have to go on is what everyone else knows. The man responsible is the man pictured in the second sketch who is between the ages of 18-40 but may be younger than his true age. I do not see 58 years old in that second sketch and the person who may have information but not be sure about it is probably going to think the same thing, especially if they are thinking about turning in their family member. There are others too who do not do it because of denial, but I wonder if that is the case in the Delphi murders.
We also have to remember that not everyone has a deep family history of aunts, uncles, nephews, and nieces. In certain cultures this is seen as completely normal. Not everyone has this in their lives.
But like so many ideas and theories, you cannot begin to discuss their veracity until the case is solved.