How do prevent someone from getting hurt?

if you know that someone in your family is busy with things they are not suppose to do,what will you do.she is totally underage but act like 18 already.the worst thing ontop of it is that her mother does,nt want to know nothing about her.she things she is so innnocent.the people that this young girl,hangs out with are dangerous.this is totally effecting me,bec i,m family