The baby needs her mother to be locked away so she cant use. The baby already probably has brain differences. We adopted a child whose birthmom was on crack and it showed up in his system when he was born and she lost her rights to him. The child, now adult, has autism and it could have been worse. He was lucky because autism is something you can treat.
The girl has to be forced to quit even if that means jail or the baby will continue to keep using the same drugs her mother uses. I kkow. Its hard. But Baby may develop the organic brain disorder of fetal alcohol syndrome which cause lifelong, irreversible serious cognitive and behavioral issues. There is not much you can do to help Fetal Alcohol Syndrome (FAS) except wish her mother hadnt drunk alcohol. Any amount of alcohol while pregnant can cause this. I would focus more on somehow getting her to a place where she can not drink or take drugs, like jail, then worry about your son right now. They could both lose their parental rights,.
My sons birthmother lost all five of her kids. They were all drug affected. The grandma took four of them but was too overwhelmed by so many kids with special needs...she felt unable to take our son.
If you feel you want to or may have to apply for custody it is a good idea to form a relationship now with CPS. The longer this woman takes drugs, the worse it is for the baby. Your granddaughter is at stake. Please think about the baby first.
Light and love!