A theory of mine is the reason for the text at 1am on said date is the time the parents decided they needed some sort of alibi after burying her body, trying to cover their tracks, Hide their crime. Yes I expect the murder did take place on the 18th but they didn't think to use BWs phone as a false alibi until a couple days later.
I don't think the brother played a part in the murder, or even knows about it, but I do believe the brother knows of the emotional OR physical abuse his sister was suffering so in the back of his mind knows what has happened but feels a huge amount of guilt so chooses to believe she has run away as it's easier than facing the truth. If my theory is true then it's terribly sad