Being an evil parent and taking away their toys is underrated. Especially if she's putting herself at risk.
Taking dangerous toys away from children who don't comprehend them isn't necessarily fun, but nobody ever said the right thing to do is always the easiest.
OK, I'll say it - blood relative (cousin) or not, the wrist slitting, car-jumping, pill-popping 15 year old mental case needs to be kept away from the 11 year old - she's not just a bad influence, she's dangerous to that child!!
Barbara, SW, & HankB have the right idea(s). We, as parents, are to
parent, not
befriend, our minor children. Sometimes that means being the hardazz & stirring the ire of our kids.
Nobody would be so foolish as to allow a minor child unsupervised access to a computer with internet connectivity in her own room? Access ought to only be in the open (like a living room) with a splitter in the video cable. One cable goes to the monitor in front of the keyboard & one cable goes to the monitor mounted higher up for Gawd & all to see.
Also, a firewall/logger tool is indicated. When objectionable sites are discovered, the person with admin authority blocks incoming packets from that site.