Welcome, Aimeelouise.
From what you have written in your post, I think the main thing I can say is to make sure that if you are going to issue an ultimatum to your mom that you must be ready to carry it out. Most here use boundaries instead. I think of it like a personal security system - what you will or will not do to take care of yourself. For example, I will not get in a car driven by someone that has been drinking - I will drive myself or find another ride.
So, you've told your mom it is you & the baby or the alcohol. What you can do now is take the steps so that you can carry that out. Do you have somewhere else to go to stay? Do you have a job so you can support yourself & the baby? You have about six months before baby arrives, so that should be your focus - setting up a self-sufficient life for yourself and the baby. You cannot change your mother, but you can change yourself and your situation.
Please keep reading & posting here. There are very helpful stickies at the top of this board & there is a board specifically for adult children of Alcoholics/Addicts. Have you tried Alateen or AlAnon? You & your sister may find it very helpful. I'd also like to tell you the Three C's - I found these so helpful when I found SR & Alanon - You did not Cause it, you cannot Control it, you cannot Cure it.
Again, welcome. Glad you found us. Others will be along with ES&H soon.