Most times “foul play” is perpetrated by someone known to the victim.
In the interview, it sounds like the brother was driving from home to Auburn with Aniah and offered to drive. Is it possible, that the further laid back seat could mean that he drove, and is it possible then that he got into an accident while she was sleeping. She woke up upon impact and was freaking out. Maybe he was high, or drunk, and she was indicating telling on him. This led to a panic and covering up a heat of the moment reaction to her anger. This would somewhat explain a relatively similar pattern of communication as he would know how she interacts in conversation, and if it wasn’t her sending the Snapchat messages, then it would have had to have been someone who knew how to unlock her phone (given it wasn’t forcefully done with her finger while unconscious), which he maybe would have had knowledge of a password.
Just speculation and theory.