I'm really really confused can somebody please help me on what I should do :(

How old is the cousin? If your girlfriend is upset about this and you care for her then you should give her all the support you can. Only talk about the incident if she wants too. Do not judge. Would it help to involve the police. To take this step is very serious so both need to think about it very hard. You need to make sure it is the way forward for your girlfriend.

Hello md:

Tell her parents.

excon

This is a serious accusation and not something a 13 year old should be dealing with, you will need to involve an adult and let them take it from there and help your friend... in the meantime just be a friend as best you can. Oh and well done for seeking advice and speaking up :)

Encourage her to tell someone like her parents, and for now talk to your own parents. Let them help you decide what you should do.