This is would involve not only an active alcohlic but your sister and her baby.
If you go to Al Anon or ACOA or read about that you learn there if basically nothing that stops an alcoholic bent on drinking to destruction.
What about your sister? If you decide to keep sending the money you need make sure it only goes to her. How is your mom getting enough money for the liquor which is expensive.
Is there any way your sister would consider or you would her moving in with you?
Or is she working on getting something going with her situation? Has she contacted any resources for work, school, daycare (leaving the baby with the mom as babysitter is not a good idea).
Yes, it is absolutely awful the downhill slide that goes on. Watch how your relationship with your sister goes. It could be or quickly become dysfunctional. After all the idiocy I have gone through with my family I have little/no contact. And my sisters were my best friends.
Have you attended an Alanon or ACOA meeting? Your sister should attend some of the meetings also. You need to put yourself first and start a recovery of your own. And you are also pregnant which makes this even more important.
Also, check out the stickies - read the roles of people in a dysfunctional family. And how rescuing is a problem.
You have no leverage at all with an alocholic, and using help with money does nothing except divert funds to the alcohol. Don't talk with your mom as much or go no contact and learn detachment (very difficult but gets easier).