I'm going to tell my parents my sister went to mexico
Why would you tell your parents about your sister?
I want to help her with her situation, and even though I think I betrayed her I think it's for the best.
If I were you in your situation, where I think someone is in danger and I may need to break a trust or something that they said to me confidential so that they can get the help that they need, I would be doing the same thing as what you were doing. However, I would recommend that you speak with your sister first if she really wanted help from your parents. Your sister has put trust on you, and why do you not just talk about it with her instead of going directly to your parents?
I did talk to her but I am still going to talk to my parents too.
It is best to have a relationship of trust with your siblings and not go behind their back to talk to your parents.