In a court case between the Capulet and the Montague families trying to define who is guilty for the deaths of Romeo and Juliet, how can I defend...

With regard to defending the Capulets, you could argue, first and foremost, that it was the Montagues' fault that Romeo and Juliet met in the first place.  The Capulets were simply having a party, and Benvolio and Romeo (both Montagues) made the decision to attend the party uninvited.  That Romeo then followed Juliet back to her bedroom balcony only cements his blame.  Had Lord and Lady Montague been keeping an eye on their son as the Capulets were keeping an eye on Juliet, Romeo would not have been free to attend a party at the house of his sworn enemy and meet the Capulets' daughter.  It was that fateful action that brought about their ultimate deaths.

You could also argue that it was the Montagues' request that the Prince not kill Romeo, and the Prince's ultimate decision to banish Romeo instead, that likewise sealed Romeo and Juliet's fates.  The Capulets justly argued for death, as Romeo had killed Tybalt.  Had the Capulets' request been followed, Romeo would have died, but Juliet might have been alive at the end of the play.  Thus, while the Montagues have several actions to answer for, the Capulets are innocent of the double suicide.