Found out that my sister is sexting ana sending nudes to somone?

so i was holding her mobile and then a message came so I was wondering what its and I was literally shocked!! By the way she's sexting and tht he asks her to send more nudes I really don't know what to do? She doesn't open to me or tell me anything about that aslo that's the first time for her to get attached to someone like this!!! And from his way of talking he's playing with her and he tells her I will marry you and stuff like that he is a big Lier
What can I do!
Found out that my sister is sexting ana sending nudes to somone?
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