Mortum Vero Eius ( Truth of the Ones Death)

There was a girl named Austine Alvarez, she has a twin sister named Antonette Alvarez.
Right on their graduation, her twin sister Nette, was found dead on the back of their school's restroom.
They couldn't find out who the culprit is
Few of their classmates and friends knew, or saw what was thw truth about Nette's death but they couldn't tell anyone about it because the culprit sent them death threath and messages , they are afraid that they might die like what happened to antonette. Austine didn't give up to find the truth about her sisters death because if she does, her twin sister nette, is giving her nightmares,warning, and clue about the culprit. Then the summer vacation Ends. Right on their school year, one of austine and nettes friends dies each time they attemp to reveal the mystery of Nettes murderer.
"Keep your friend close, and your enemy closer"
They saw the note written on Nette's Journal.
What does that note means?
Was it a clue?
Was the killer is one of their friends?