I found porn on my 12 year old sisters phone.?

Okay, so today I went up to her room and, I knocked on the door, it took her awhile to open up the door (it was locked). When she came out she was wearing underwear and a t-shirt. I had a feeling she was masturbating, so when she went downstairs i looked through her phone. I saw 3 videos from porn hub. I didn't... show more Okay, so today I went up to her room and, I knocked on the door, it took her awhile to open up the door (it was locked). When she came out she was wearing underwear and a t-shirt. I had a feeling she was masturbating, so when she went downstairs i looked through her phone. I saw 3 videos from porn hub. I didn't know what to do so I put it back and went downstairs. She's only 12 (I'm 15) and I understand its "natural", but were religious. She shouldn't be masturbating. I really want to tell my mom, but my sister has told me before that she has something really bad on me. I have no idea what it is, but she keeps dropping hints. I really just want her to stop watching porn, and understand what sex really is and not what the "movies" make it out to be. She is very promiscuous in the way she dresses, and I dont approve. My mother doesn't have a problem with the way she dresses because she sees her as her little girl. What should I do? I was thinking about taking her phone, and plugging it into itunes, block safari and downloading a children safe internet browser. I'm just scared of what she will say.
Update: I wasn't spying on her, our tutor wanted to ask her a question and she wouldn't answer so, i went upstairs to check on her.
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