I also wouldn't be surprised to find out that all the various reports (911 calls, etc.) where they claimed the brother was acting out that he may have been frustrated and agitated himself with not getting answers from his parents on what happened to Erica.
If the son Jamie truly believed that Sandy and Casey killed Erica and buried her in the back yard, then, he himself, could have been living in fear, paranoia, and suspicion of his own parents. And especially as time went on when Erica's bedroom was no longer her bedroom.
Because even "IF" Erica went off to live with biological relatives doesn't mean that she and/or her other siblings (whether they were related or not) that she grew up with should have been isolated from each other. And Jamie was 18 at the time Erica disappeared so he may have been pressing his parents for a phone number and address to visit her. And that constant, "we don't have a phone number or address where Erica is" probably did nothing more than build his own suspicions that Erica was probably dead.