In Act 3, scene 1, after the brawl in which Tybalt is killed, the Prince arrives on the scene, and upon witnessing the carnage, makes his relationship to one of the morbidly injured characters clear. He says:
I have an interest in your hate's proceeding,
My blood for your rude brawls doth lie a-bleeding;
In this instance we are not exactly sure who he is referring to. Tybalt or Mercutio? We only discover who it is in a later reference. The reference is made in the Prince's speech at the end of the play:
And I for winking at your discords too
Have lost a brace of kinsmen: all are punish'd.
This is said after the discovery of Romeo, Juliet and Paris' bodies in the Capulet's crypt. We can only conclude that the "brace" (two) the Prince refers to, are Paris and Mercutio, since Romeo is not related in blood to either Mercutio, Paris, Juliet or Tybalt.
Although Juliet is related to Tybalt who had also been killed, we can correctly assume that the Prince is not referring to her, since he would have used an alternative means to refer to her, rather than include her in his reference to 'kinsmen". Also, there are no references in the play as to her being related to the prince or that she is of royal blood. We do, however, know that Paris is royalty, since there are numerous references to him being a count or 'county'. So the reference can only be to him in this instance and Mercutio in the former.
The Prince's two kinsmen are therefore Mercutio and Paris.