The worst part for the family is that the media can label their father a rapist and murderer without him actually having been convicted. The police have strong evidence, from the sound of it, but with a living person the news media wouldn't be allowed to go around saying unequivocally "he's the guy" without a guilty verdict.
The upshot is that even if there's been a mistake, this man's name will never be cleared, and his family will just have to live with this.
The police going to ask them in the first place was a wrong decision. A family minding their own business doing nothing wrong should not be burdened with someone showing up at their door saying "we think that you're the children of a murder rapist. If you don't help us that's definitely what we're going to conclude. If you do help us there's a chance that we'll conclude it anyway, but you never know. He's dead now so we have no requirement to overcome a legal defense. What say you?"
Are you saying you wouldn't like to know if your father were a rapist-murderer? I don't get it, really. And I'm pretty sure it can help those people learn better about themselves and their past. This is why they accepted.
> The family agreed to help the police figure it out.
No. A couple of children, grand children, or cousins agreed to share their DNA (or it was legally removed without consent). The rest could have told the cops "NO", and they still would have gotten enough DNA evidence.
This. Detectives’ thirst for solving (or just finding someone to blame/force a confession out of) crimes will often override their duty for true justice, by whatever means they want.
It's rather important that we say "accused's family". It's irresponsible to label someone a murderer without giving them an honest opportunity for defense.
The upshot is that even if there's been a mistake, this man's name will never be cleared, and his family will just have to live with this.