I can relate, OP. I had the good fortune of acting as my girlfriend's date when her cousin, the bride, got married. The groom seemed nice enough, but his friend, let's call him Joe, turned out to be a total asshole.
Joe was dancing with my girlfriend's older sister, let's call her Jane, for quite a while. It seems the open bar was a little too much for Joe to handle, and in his drunken stupor he started grinding up on Jane. This made her very uncomfortable, and though she asked him to stop, he refused. She ran to the bathroom moderately upset, probably because she felt embarrassed for having Joe do that in front of her family. Joe chased Jane into the ladies' room, and, from what I heard later, was calling calling her a bitch and other such names. Someone alerted Jane's, also my girlfriend's, father, who rushed into the bathroom. He tried to calm Joe down and make him leave the bathroom, but Joe replied "WHY? 'CAUSE YOUR DAUGHTER'S A SLUT??"
Shortly thereafter, Joe was thrown out of the bathroom by Jane's father and kicked down the hallway. Joe's two friends, including the best man, all of whom were drunk, came to his aid. A few of the bride's relatives, Jane's father, and I fought and subdued them. Luckily, the wedding reception in another part of the place where we were had security, who were happy to escort them outside.
Funnily enough, my girlfriend's/Jane's father had despised me until then, has been a little nicer to me after we fought side by side.