Should I tell my sister about her daughter's instagram?

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Alright. My sister went through some rough times in high school, and ended up pregnant. They stayed together for 8 years, then her husband left without a trace, never to talk to her, or their daughter, Serena for a very long time. Serena didn't see her dad again until she was 13, but by then she really didn't care or want to see him. She spent years wanting to see him, and still wrote him letters almost every day

My sister was never home very much due to her job, and Serena spent most of her childhood with her grandparents. She used to get bullied in school really badly, and was molested by her doctor when she was 10.

Serena is 14 now, in 8th grade.

I recently got an instagram, and checked if Serena had one. She did. I clicked on it, they are all very very provocative photos. Many of them were of her kissing her boyfriend, who is a JUNIOR in high school. THere were a few pictures of her smoking pot with a bunch of guys, her sticking her butt out in tight yoga pants with her new "friends". I looked at her "ask fm" attached to it and she basically admitted to having sex with her boyfriend, a few other boys, getting drunk, and high.

Should I tell my sister? Is it really my business?
ALSO, she's gotten in legal trouble before, she's shop lifted beer