In an ideal world justice would be 100% accurate. It can't be, ever, but fortunately our system is the best available and the incidents of error are thankfully minute. There will be mistakes, but the idea of sentencing based on the verdicts not being accurate is even more flawed and destructive to the prosecution of crime.
As far as extended family of the criminal goes, if they are humane they will only feel for the victim and understand why what's being done needs to be done. In a crime there is only one victim and that's the person who was killed or savagely raped by the guilty party. The criminal and his family are not victims, and the guilty party hopefully will die with genuine remorse for the victim(s) of their crime(s). If not, all the better that they are being killed for what they've done.