Heredity cannot be overemphasized. Thirty percent of the children of alcoholic parents become alcoholic. Girls from families with serious problems are certainly more at risk than girls who come from healthier families. But I don't want to overstate this. Teenage girls from well-adjusted families sometimes develop serious problems with chemicals.
Peers play a role.
In general, kids whose friends are heavy users are more likely to use, while kids whose friends abstain are more likely to abstain.