I don't want her in my house

[Sephine]

Delay the party, on the grounds that you wish for the mom-to-be to enjoy the party and for everyone to have a chance to know what sizes to get the new baby. It is a common enough thing to have a baby shower after the birth of the baby, because a lot of family loves passing the baby around. To be germ safe, that is best done when the baby is a few months old. Mom is recovered enough to enjoy it, and cousin will have gone home.

[cicero]

I don't know what the etiquette rule is.

But i think that if you don't want this person in your house (and you have good reason for this), then you don't have to have this person in your house.

now - this does mean that you also have to deal with whatever fall out there might be. you need to be upfront, clear, and firm with you SIL tell her "I want to have this party for you but we will not have X and Y at our house. so it's up to you - If you need to have X and Y at the party, then I am going to have to back out of hte hosting. if you are ok not having them, then i will carry on." you might add that you will do your best to hold the party after cousin leaves town.

[RooRoo]

Unless it is only known to you - which I doubt - I see nothing wrong with saying that you do not want them in your house because they are alcoholics and you don't like their behavior.

That makes a distinction between rejecting persons, and rejecting (alcoholic) behavior.