What will Juliet's kinsmen do if they see her talking to Romeo?Specifically asking in Act II
Once Juliet has figured out who Romeo is, she says:
"The orchard walls are high and hard to climb, And the place death, considering who thou art" (63-64).
Juliet worries that Romeo may be recognized and killed by Capulet guards. As a Montague and enemy of Juliet's family, Romeo is literally risking his life by climbing into their domain.
"If they see thee they will murder thee" Juliet reiterates on line 70. Romeo's next two responses (see lines 71-73 and 75-78) show that unlike the practical Juliet, he is acting reckless, as if he is so excited that nothing could possibly harm him.
It is useful to note that when Romeo says he'd rather be killed by the Capulets' swords than by waiting for Juliet to love him, it is one of several occasions that the couple profess that they prefer death to separation.
I think that you are probably asking about what Juliet says in the famous balcony scene in Act II, Scene 2. If that is what you are asking about, she says that her relatives will kill Romeo if they find him there. She tells him that twice in this scene. She says that he will be killed
considering who thou art,
If any of my kinsmen find thee here.
I am not sure I believe her since her father did not want Romeo killed, or even confronted, when Romeo snuck into the Capulets' party. On the other hand, I suppose that in those days they might have killed anyone (Montague or other person) who was in their walls and talking to their daughter.