Oglala Sioux v. Fleming (previously Van Hunnik) is the class action suit initiated by the Oglala Sioux and Rosebud Sioux Tribes and the ACLU/Stephen Pevar arguing that Pennington County, SD is violating the due process rights and ICWA rights of Indian families. The case is complex and on-going. Since the suit was originally filed in 2013, the tribes and individual tribal members represented in the class have strung together a series of compelling district court orders in their favor. Recently, the state appealed those orders to the Eighth Circuit. You can find those orders, and some of the briefing collected here.
Here is the current briefing in the Eighth Circuit: