There's the legal way which is to call the police and let them handle it. In many jurisdictions the abused spouse doesn't get a say in whether he is arrested or not. This takes the burden of responsibility off of the victim. If there is any visible physical evidence of abuse, he gets arrested and an automatic restraining order goes into effect.

While I don't endorse or encourage this, the other way is to get a group of gents to find him alone someplace private and give him a dose of his own medicine, all the while telling him that they heard he likes to hit women. This can backfire in lots of ways though.

I can guarantee that if YOU go after him you will get arrested. Guys like this will use the system to their favor when they feel threatened. Be wary of being alone with him too. Since he knows you carry, he could lodge a pre-emptive strike against you by calling the police and claiming you pulled your gun on him. I know it's your sister, but is it worth giving up your freedom or gun rights for life (assuming you get nailed for a felony)? Also keep in mind that according to CO law, he can have you charged with felony menacing just because he fears you - nothing more.