If one identical twin commits murder, but the police cannot prove which one, then legally both must go free. This is a mistake. The purpose of execution is to improve the gene pool. Frost and Harpending (2015) show that by executing "between 0.5 and 1.0% of all men of each generation... the homicide rate plummeted from the 14th century to the 20th."
The correct solution is to execute both brothers.
What about families? Immediate family share 50% of the murderer's genes. Should they be executed? If this is too small a connection, what should the threshold be. 100%? 75%?
(In reality, identical twins have different fingerprints and slightly different DNA.)