In Scene 5 of Act III of "Romeo and Juliet," Lady Capulet enters Juliet's chamber to find Juliet completely distraught. She wisely remarks upon excessive grieving for a loss loved one:
Some grief shows much of love,/But much of grief shows still some want of wit (III,v,74-75)
When Juliet asks that she be allowed to weep for such a deeply felt loss, Lady Capulet, in a moment of dramatic irony, retorts that Juliet
weep'st not so much for his [Tybalt's] death/As that the villain lives which slaughtered him.
While Lady Capulet believes that Juliet cries because the murderer still lives, she does mourn Romeo, but in a different way:
...I do with all my heart,/And yet no man like he doth grieve my heart (III,v, 82-83)
Juliet grieves that Romeo is banished and that he has committed the murder, not that her family has avenged itself upon him. She then asks her mother for poison so that she could give it to Romeo, but she really desires to kill herself.
Then, when Lady Capulet announces news that should cheer her--that she is to marry Paris--Juliet's reactions are not what she expects. The daughter tells her father that she will not marry:
"I'll not wed, I cannot love,/I am too young, I pray you , pardon me. (III,v,187-188)
This, too, is dramatic irony since only Juliet knows she is already married to Romeo. Likewise, when Juliet begs her mother to "cast not me away" (III,v,200), saying she will make herself a bridal bed in the tomb with Tybalt and her mother rejects her, "I have done with thee"(III,v,205), there is much irony of situation as Lady Capulet unwittingly predicts what will happen.
Lady Capulet thinks Juliet is crying because of Tybalt's death. He was her cousin. Juliet is actually crying because it was Romeo who killed Tybalt, and as a result, he has been banished from Verona. As a result of thinking Juliet is crying over Tybalt, her mother tells her she's going to send someone to kill Romeo. Then her father thinks having Juliet marry Paris right away will cheer her up.