Answer:
She thinks both Romeo and Tybalt killed each other
Step-by-step explanation:
Juliet would obviously be confused since the Nurse can't really be clear:
O Tybalt, Tybalt, the best friend I had!
O courteous Tybalt! honest gentleman!
That ever I should live to see thee dead! 1785
Juliet responds:
What storm is this that blows so contrary?
Is Romeo slaughter'd, and is Tybalt dead?
My dear-loved cousin, and my dearer lord?
Then, dreadful trumpet, sound the general doom!
For who is living, if those two are gone?
She seems to think that they both died fighting each other