First off, welcome to the forums BC!!!
Second, there are many ways this could have been done better (monday morning quarterback time). The Defender shouldn't have drew his sidearm unless he really intended to use it. Understandably, the drunk is, well, drunk and not in his right mind, but still extremely dangerous. After all, he did punch the Defender who was holding his sidearm.
Lots of coulda shoulda woulda in this scenario. I'll give you my take: At the onset of trash talking, immediately tell the sister to get back in the truck. Tell idiot friend to stop fueling the fire, and get back in the truck. Drive away. If idiot friend starts a fight with drunks, well, that's his stupid fault, he should know better. He's already outnumbered 4-1 by drunks which is NEVER a good thing, armed or not. Oh, btw, a gun is NOT a club when it's loaded. One should only use a gun as a club when all ammo has been expended and the threat remains.
Summary: Sister and friend are stupid for fueling the fire, and getting too close. Defender is stupid for not getting sister and friend away from drunks, pulling a gun, and using it as a club. This guy needs tactical/strategic defense lessons.