To answer your main question, no, I don't see it that way.
What you are doing is setting a boundary and letting him know what that is, which is rather polite of you and let's him know what the xmas plans are if he chooses to drink - which he absolutely can - but he is then not invited to join you, nothing wrong with that.
Forcing your will would be saying you will do this and you won't do that - you aren't doing that, you are telling him what your intentions are.
The only place I think it gets a little murky is handing him the bank card and then stating if he chooses to buy alcohol etc. One really doesn't have anything to do with the other. What he chooses to spend money on is out of your control - so it's two separate things.
Now, all that said, the rest sounds like the xmas from hell! You having to deal with your family, with SS's Mom (you mean you are spending xmas day with her as well or just SS?).
Why are you doing all this? What xmas would you like? If you would prefer to stay home and just cook a turkey and watch movies, why don't you do that. I guarantee you that you do not get extra brownie points for attending functions you don't want to be at.
Plus, what's the point?