Whether the case is resolved probably depends either on a confession by one of the parties or the chance discovery of the body.

These are not sophisticated criminals, so the body either went into a dumpster or is buried in a shallow grave somewhere. If it is dumpster, then she is now in the Apex landfill. Detectives, if they were smart at the time, have already determined where trash was being deposited at the time of the disappearance, but they would need some strong indications from other evidence before they search.

If it is a shallow grave, these bodies tend to turn up eventually, dug up by coyotes or exposed by erosion. With a body, you can start to make a case. Again, these are not sophisticated killers, so they probably weren't as effective as Jimmy Hoffa's.

Even if the body is never found and no one is ever charged, we can still expect the perpetrators to live short and painful lives, since guilt wrecks havoc on the immune system.

Ultimate responsibily lies with a system that has no real way of filtering good foster parents from bad. Nearly everyone who completes the training and application process gets a foster license. The system is so desperate for foster parents, that they'll use just about anyone.