14 year old sister sexting help me !

I need some advice I found my old iPhone in my 14 year old sisters bed today ( I replaced mine with a 6s plus) I realised that she stole it from my bedroom had known to unlock it by using my PIN number and then found her sexting to older men. The things I read were horrendous things like fucking in a park and having to take a pregnancy test and then sending pics to others and all the graphic stuff they would do and also her looking up porn gifs on my browser I'm absolutely gobsmacked. I thought we had such a good relationship and trusted each other but turns out I was completely wrong. I confronted her In front of my mum about her stealing the phone but I've only just gone on it to see what she was doing and came across her conversations on Snapchat!!!!! It's put me in such an awkward position please someone give advice on what I should do??