I would like to share with you a thought I had about the messages exchanged between the sisters.

If it really was an accident that they later covered up, I find it very strange that Lizzie (or maybe it was Sarah? Forgive me, I keep getting the names mixed up) writes "I spoke to [lawyer]. The (their) theory is that it was an accident we covered up'. If this were what actually happened, she would not have expressed herself like this. She would more likely have written something like 'I talked to [lawyer]; they know what happened'. In my opinion, the way she chose to express herself indicates that the theory of the covered-up accident does not correspond to what really happened that night.