First welcome,
I am so sorry you are in such a difficult situation...my concern here lies with you and your 3 "brothers". Being raised by an out of control alcoholic and potential addict is emotional (and perhaps physical) abuse.
About helping her..there is really nothing you can do to get her sober short of locking her up. If she using while she is pregnant, I am sure that down deep she already knows she is threatening the life of her unborn child. In todays world that is a crime... are you prepared to report her to protect the child?
The first thing we all learn is that we are powerless over the alcoholic no matter how out of control the situation is. They have to want to clean up their lives themselves. We do have the power to take care of ourselves and protect ourselves if needed. Husbands steal from wives, son's from mothers and mother's from son's...sad but true. I lived with everything locked up when my A was at home. I have called the police when violence erupted (my A is my son)and I pressed charges.
Keep posting perhaps on the Al Anon or Nar Anon boards...tell us more...are you all living with her? Where is the father/husband?
Hugs,
JT