Now is the time for them to get an ATTORNEY familiar with the Family Court System and who can get into Court NOW and get a Guardian Ad Litem appointed for the child.
Sounds to me like a Social Worker that enjoys her power and the 'hell' with the kid. Tell Bro and SIL to keep doing what they are doing and to fight like 'hell', including going to this 'workers' SUPERVISOR. If nothing else the Supervisor can assist in getting the case assigned to another worker who is MORE KNOWLEDGEABLE about addiction and recovery.
Reassure them that this is far from over and a good attorney can really do wonders.
Sending prayers for you and your family.
Love and hugs,