I'd like your opinion on a disgust/sick matter!

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Hey Guys, I'd like your opinion on something, comment please if you have the time.

Okey so I'm getting the blame for protecting my sister-in-law (or soon to be sister-in-law) because I'm protecting her from what could be a paedophile on Facebook and DA. Now she is 13 years old and she was dating a 16 year old on-line and this "16 year old girl" thinks that it's okey to get a 13 year to send her naked pictures of herself.
I have proof that this "16 year old girl" admits to asking and bring up the idea of asking the 13 year old to take these pictures, I was the one that found these pictures on the 13 year old sisters phone and what would you do:
Tell her mother about the pictures?
Ignore them?
Or just forget you ever saw them?
Well I chose the obvious one, I told her mother and this "16 year old girl" thinks that I'm in the wrong for telling her mother even though taking these pictures and being in possession of these pictures is against the law, now I've never done anything against the law because I know what's right and wrong.
I've looked on the internet for if these things are actually against the law and it says "Yes, it is", It's something called "Sex-ting".
Now I think this "16 year old girl" is sick and what she has made the 13 year old do is gross. The 13 year old is now mentally hurt from all this because the "16 year old girl" claims to love her and made the 13 year old feel special, now if anyone made you feel special and said they loved you you would never take naked pictures just to make someone happy which this "16 year old girl" claims that is what the 13 year old was doing but if you truely love someone you would NEVER make someone do something as sick and indecent as this.

Please comment with you opinion, I'd like to know what everyone else thinks.
Thanks xxxx

Lauren xxxx