Whoever knows is around 80 years old.
If there were only one person who knew what happened to Ron Tammen that night, I think it's more likely that they would confess than if there were a group of people who knew. Loyalty to a small group is more powerful.
The problem with confessing, of course, is the legal consequences. And recent events have shown that no one is considered too old to be safe from prosecution for crimes 60+ years in the past.
Which cuts both ways. No one should commit murder and then think they are somehow safe from the consequences of their deeds. But there are cases like this, where the most likely explanation was not malevolent but just plain stupidity (dumping a frat member out in the countryside during cold weather) where punishment seems pretty irrelevant after so long.
My sense is that Ron Tammen's sister would just like to know what happened and is not particularly interested in vengeance. Unfortunately, it is not up to the victims of a crime to determine the consequences.