I had originally thought that the 7 year old left the toilet flushing note and the 13 year old got offended, but now that you mention it, maybe it was the other way around!
Yes, the 7 year old seems to have been the cuddled one and jealousy may have been brewing.
Tragic all around, and it’s too bad it couldn’t have been avoided, but the parents claim there were no red flags and that the 13 year old had never been in therapy and never been in any kind of trouble at school or elsewhere.
But yes, communication problems had to have been part of the dynamic which broke down horribly.