Can a baby’s father take him away from the mother if she has a bench warrant?

my sister is in an extrememly abusive relationship & has been for the past 5 years or so. he’s mentally and physically abusive, which was documented once at the hospital when he bruised her ribs and face. they recently had a baby & he’s 9 months now. they have both been living with the me & my parents since february. the father does virtually nothing to care for the baby. he’s a severe alcoholic. he works one or two days out of the week but has to travel out of state for it. he is now threatening to take the baby away from my sister. they aren’t married, which is good, but my sister has a bench warrant because she failed to show up for probation on a misdemeanor marijuana charge. he keeps threatening to have her arrested and take the baby so she can never see him again. she wants so badly to escape him, but he says he will kill her and my family if she takes the baby. he had a child like 8 years ago with another girl, and she took him & moved away bc of his abuse. what can we do?