She isn’t reliable, even to Juliet. The reason is, the nurse doesn’t even discuss the problem that Juliet’s parents want her to marry Paris. Instead she is trying to convince her to marry Paris even though she knows that Juliet is Romeo’s wife. She could’ve consulted with Friar Lawrence about this or tell her master (Capulet) to delay the date a little bit so she could have time to think about the marriage. Juliet doesn’t tell her about the plan because she doesn’t trust nurse with her secret or she didn’t expect any help from her (nurse). When Tybalt was murdered by Romeo nurse was telling that men are unfaithful and cheaters which referred to Romeo. These things made Juliet think that nurse is dishonest and she doesn’t share her plan with nurse so that she could do it properly but in the end they end up dying instead of living happily ever after.
Romeo and Juliet’s tragic death is because of the nurse’s ignorance. Their deaths show that their love is pure and innocent because they didn’t want to live without each other. But because of nurse’s attitude they couldn’t get any help from her. The nurse could’ve told the Capulet family about their relationship so they could stop it, she could’ve stopped them from marrying and if she wanted to help Juliet she could’ve been more reliable and honest to her thus the story could’ve ended