I sincerely hope you all have a nice weekend and that he manages this social occasion without alcohol - and can do so pleasantly. You deserve to have a nice weekend with your family. It is important to keep your support network strong.
The best predictor of future behavior, is past behavior. Having a plan B is not about expecting the negative, it is about making plans to enjoy your weekend no matter what he decides to do. I make all kinds of plans when I go away for a weekend. When I was married to an alcoholic, I also planned for that. Well, not always, but I learned too. Except I wasn't very smart because looking back my plans didn't protect me and my life/joy, they protected him and his drinking. That is a very subtle, yet very slippery, slop.
So I am here, posting to you, because I wish I would have been here and gotten that message when I was in the same spot as you are in now. i didn't find SR until 16 years later when I was leaving. Make plans that protect you and your joy, not him and his drinking. You'll never regret that.
For instance...I would leave early, I quit planning things that were not compatible with his drinking, I quit hanging out with people that were down on his drinking or my marriage, he irritated my friends because he was hard to talk to when he was drinking - so I quit doing things with my friends, I quit planning boating activities because I didn't always feel safe, I quit planning family beach trips because most of his time was spent driving back and forth to the gas station for more beer and I just got irritated, I quit doing social things that involved alcohol because he over did it, blah blah blah.
We were together for a long time so we did have good times of course, but drinking got in the way a lot too. As the alcoholism progressed the good times faded away. It changed us both.