This is a hard one for which I do not have a lot of advice but I had custody of a pair of siblings for 30 days (unrelated) because their parents got DUI's (they were divorced) within 3 days of each other and CPS took the kids away. The father ended up with custody and mother had only supervised visitation for one hour per week because she would not agree to using soberlink.
This would probably be something to take up with a social worker or a family advocate. The mom's shouldn't always get the kids back just because they are the "mom". What does your daughter want? She is old enough to have some input in her safety.