Hi Chloe ((((hugs))))
I'm so sorry you are going through this but it is very similar to things all of us have or are going trough.
A little help with terminology. Use AS for the alcoholic sister.
One of the big things here to keep in mind is the 3 c's.
You didn't cause it
You can't control it
You can't cure it.
Take that to heart. It really helps keep things in perspective. As others have said boundaries are to protect you. So use them to keep yourself as free as possible from the madness that is going in with your AS.
It is also ok for you to set up boundaries with your mother as well. She will stop with the codie behavior when she is ready.
I have no experience to offer with your nieces. Where is their father in all this?
Come back often, we are here to help.
Your friend,