The one 13 year old invited the other 13 year old over to spend the night. The 11 year old sister probably got along better with the 13 year old invited over. Could be they made fun of, or else the camaraderie between the 11 year old sis and the 13 year old visitor invited jealousy and rage.
I have no clue what happened here, I am just guessing. Pure speculation. No matter what, this is a sad thing to happen.
That is a reasonable explanation.