So the picture that's now emerging is that you are uncomfortable about having hour husband's son around at Christmas because he dislikes the way you both drink. Possibly (though admittedly this is just my speculation but it's a picture I've seen before) he feels you make his dad drink more than he would do left to his own devices.
The step son has indicated previously, whether directly or not, that he's uncomfortable being around the pair of you when you're drunk, you sense this and feel judged.
Well, I'm the husband's son in this situation. I was profoundly uncomfortable at Christmas every year with my dad's drinking. I found it boring and humiliating and thought he looked like an idiot. I'm actually still pretty traumatised, in my early 50s, by my dad's drinking.
So the reason you don't want your husband's son around is because he holds a mirror up to you and reveals what an embarrassment you are when drunk and the fact that he is clearly uncomfortable with it shows you feel guilty.
You're being incredibly selfish. And if you value your marriage, I'd give a bit of thought to the unintended consequences of this behaviour for your husband and his long-term relationship with his son. Because if he distances himself from the pair of you, your spiteful and selfish attitude will have been a factor here.
You and your husband can get dribbling drunk and make twats out of yourself if you really want to. You can choose to do it on a day when his son isn't there. Be a grown-up and deal with it.