Your brother needs to attend Alanon. He can make sure that your niece is not sucked into the cycle that is: enabling.
Your brother, you, and your niece need to understand that alcoholism is a disease that is serious. Sick people make poor choices. In short--your nice will need to learn to call a spade a spade.
Who knows "what happens now"? Probably, your ex SIL will not change until she is sick and tired of being sick and tired. And that may not happen for a long time b/c it sounds like she has plenty of supporters and enablers and alcoholics around her to allow her drinking to continue.
You cannot lose a child because you drink alcohol but you CAN lose custody id you are neglecting or endangering the child. If your ex SIL does this, she needs to be reported. Repeatedly. Maybe that will bring her "bottom" up so that she gets the help she really needs. If it was logical, NO O. would *choose* alcohol over their child. She's sick. She needs help. When she's ready.
Oh--and your brother CAN get custody--no matter his work hours--he just needs daycare and or a babysitter. After all, that's what single moms do all the time right?
(I hope that's not a convenient excuse to leave this little girl in a situation that could be considered dangerous.)
Good luck!