We have a project in the works to send out to the reservations in the isotope areas, I think it is in the process of being updated with the new information.
It's possible the middle child's mother isn't missing; she could be a murder cold case where no one knew about the middle child to report her missing, or she could be incarcerated and has no idea what happened to her, I imagine it is difficult to get help finding someone if you're in prison. She could have also consented for the middle child to be with her father, and has since passed away of non suspicious reasons.
One missing person I'd like to rule out as being the middle child's mother and she is missing. Carlene Brown went missing from Rawlings, WY on July 4th, 1974. However, she was adopted and only has dental records available, no DNA.
https://www.findthemissing.org/en/cases/14488/2/
http://www.websleuths.com/forums/showthread.php?98727-WY-Carlene-Brown-19-Rawlins-4-July-1974