Not necessarily.
I know I've mentioned this before, the last time I served on a jury we got talking to the bailiff and he mentioned another trial he has helping with in the same building.
It was a rape case. The prosecution had DNA evidence. The defendant kept blaming his twin brother.
It was his third trial.
I read about a case like that, where the innocent brother was actually arrested several times, but luckily, he had alibis and was able to point the police to his twin as the likely suspect.
http://scienceline.ucsb.edu/getkey.php?key=244
Identical twins have same dna, fraternals do not...however there are factors that can distinquish between two, if known.