Terri is now with her mother and her father. Her parents also took her older son under their care recently, when (for whatever reasons) he needed to leave the home. Her father seemed to know investigation details we did not. He likely knew them from her mother who was there all the time. Eventually, older son ended up with this bio-father. IMO, it seems the older son did not leave with the plan to join bio-dad - but it eventually happened that he did move to his bio-dad's.
IMO, - and I'm very much speculating - we may learn Terri's mental health has been deteriorating for quite some time. And her loved ones have all been aware of this. This would explain her father's frankness in the past few weeks. This would explain the family supporting her initially as they did. This would explain the "uniqueness" of Kyron's disappearance.
All things considered, I'm still open to the possibility that Terri did not do this to Kyron - however if she did not, something she did do caused it to happen.
All JMHO, and speculation.