The “identical nature” of their relationship is irrelevant here in whether the now-confessing sibling is guilty or not and whether the poor brother should be released somehow.
Look into the legal aspect. Is there a statute of limitations. Can the now-confessing brother be legally charged or not? How alike the two are has no legal bearing according to indictment, trial, conviction, sentencing and/or freeing of the ‘wrongfully’ convicted other brother.
If the real supposed killer can’t be put on trial and jailed, then his confession is BS and his brother should stay in prison.
I don't believe there is a statute of limitation on a prosecution for murder.
As I wrote above, I think this is a case where they are simply trying to create reasonable doubt, so that neither can be convicted.
Sounds like BOTH are criminals. Jail both.