My thoughts are all over the place because there are pieces to this terrible story that just ring the alarm, but I do agree with this point of view.
Right now it is literally on film that DB took Cash and came back again, but left before he could harm Carter. He's the suspect. I don't think he was being instructed or that it isn't DB in both instances.
There are a lot of creepy parts to the narrative as we know it so far, the timing of calling 911 and so on, but I am not blaming the girlfriend yet for the poor living conditions. I know she previously lost custody of her own kids before this, as she stated, but there's something to be said for her in trying to make a home for the twins. If she was hooked up with a guy who moved in with his kids and then dumped them, that's a big ask for someone who has four teenagers already.
The room looks shabby and too easily accessible, and I think the youngest kids should be in the interior of the house or in a room facing the front. I think one convertible crib turned toddler bed is too small. But the pictures we see of the boys shows me they are clothed appropriately, they have toys, maybe that's the best she could do at the time.
I also agree she should have notified authorities about the kids being left with her. If LE comes up with evidence that she was instrumental in keeping the bio mom unaware, we will find that out.
What I think is sketchy is if the home video was sold to the DM or any other media, because when money is a factor, motives become suspect.
At this point though, I see one suspect in the kidnapping/murder and then a dysfunctional household perhaps, but no conspiracy.