First things first: put those knives back in the drawer, unless you want to find yourself behind bars before you can say 'bunchamugs'.
Second things second: Don't believe stereotypes. Anyone
you meet along the street can be a criminal of any stripe, but you can't go about treating them like they are unless there is proof.
Make sure your sister knows what to do if someone touches her inappropriately (and that is run away screaming - no kidding). And make sure she knows she should tell
your parents if such an incident happens. Many potential molesters threaten to harm kids that tell on them or convince them that nobody will believe them (especially if the adult in question is a particularly upstanding member of society).
If something does happen, don't allow her to be alone with that person again, and make a report to an appropriate channel.
Above all: Get someone to teach her internet security.
Everything about not disclosing names or locations to people she doesn't know, and especially NO PICTURES. (That's why someone else than you needs to do that.