Good Girl Turned Bad

I have a cousin who is 13. When she was in sixth grade, she was still a good girl with great academic achievement and never did anything wrong. But when she started changing class in grade seven (my school always changes classes every year), she became really strange. She started using the phone and SMS a lot, and when I wanted to see what was she doing with her phone, she always hid.

She likes playing with some bad girls at her class (one of them has been in seventh grade twice). She started badmouthing my clothes and my music style. And last week, she stole money from my grandma. When my grandma wanted to take it back, she tried to hide and answered in a strange way.

She gets worse every month. I really want to help her, but what can I do?