Wtf is wrong with her?

I'm 17, my sister (Mia) is 14, her friend JUST turned 13 (Katie) and her friends brother (my ex bestfriend, Marcus) is 15. Anyway, so I used to be bestfriends with Katie and her brother Marcus for like 5 years, I was also bestfriends with this other girl (Jessica) for like half a year longer. We used to AWAYS hang out until last year, we had these huge arguments because Marcus started smoking and stealing and Jessica also started doing it with him. I couldn't handle it anymore and one time I almost got arrested with them cause of what they were invovled with so I just decided I wasn't going to hang out with them anymore and stuff.. things ended pretty badly. My sister however still stayed very close friends Katie. Anyway, so recently Katie who JUST turned 13 has been doing some pretty weird stuff over the internet with older guys...She plays this online game and meets guys on there. Most of these guys are 14+. The oldest shes talked to I think is 16, but honeslty they could just be like 21 years old pretending to be 16. She started having phone and weird stuff like that with them when she was like 12.. Then recently she posted pics of herself nude online when she turned 13. She was crying hysterically about it with my sister and it took them hours to get people to stop insulting her online over it and she felt awful. ANyway in the end, my sister and her decided they weren't going to talk to the people anymore and she wasn't going to do it again. However, she started talking to the guys again and they started complaining like "what am i gonna do when i'm horny now?" and all this other crap. My sister asked me if we should tell Jessica (we just recently made up and stuff) , we wanted to tell her because she's REALLY close to Marcus and stuff and he could probably stop Katie from doing anything stupid since my sister can't, since he's Katie's older brother and all . Anyway, Jessica (marcus's and mine bestfriend) told Marcus who told his sister Katie. Katie I guess got mad at Mia cause Marcus knew? And now posted on her facebook how upset she is and Marcus's friends are posting stuff about beating her up for being an "unfaithful friend". but it's like wtf, we didn't go around telling anyone about it, just the people whO i t hought could help her. :\ Now I'm scared they're going to do something to my little sister but I don't think she did anything wrong, wtf. Why is Katie mad at Mia? :\