If I was your dad, I'd give you shit for your initial behavior.
Rather than relying on a gun or your dad to defuse the situation, maybe you should teach your sister to drive a bit better. She should brush up on how to judge the speed of an oncoming vehicle so as to not cut them off (it doesn't matter how fast he was going over the speed limit, he was still cut off). You guys might consider not flipping off people and yelling at them after cutting them off ( I see you mentioned that you won't be fingering or yelling at them again). This would have avoided the whole situation in the first place. Saying you're sorry to a enraged idiot after the fact is going to do fuck all IMO.
I'm not suggesting the guy was right to be so pissed over the situation (he sounds like quite the idiot), but next time you should avoid starting the whole scene because the roads are full of these types.