Should I talk to my sister's friend without her knowing?
That depends. Is there a reason you need to talk to your sister's friend?
Yes that bitch is always doing stuff behind my sister's back
You shouldn't confront your sister's friend without your sister's knowledge. They will resent you for taking matters in your own hands. Talk to your sister and explain why you think she should stop being friends with that person.
I'm going to do it anyway and get my sister's friend to confess about doing stuff to me.
It might not work out like  you intend and it will just make problems for your sister. You should at least consult her before you talk to her friend.