In these cases, Alceste, other entities need to get involved - law enforcement, department of social or human services and CPS, etc.
There are people that NEED to be made aware of the situation and the girl, who IS NOT YET an adult should not be going at this alone. Legally, in the US, an adult is responsible for her care. If those responsible have abused her, there is accountability. Someone has to step up and blow the whistle.
If she is mature enough to make a decision to purchase over the counter emergency contraception - she's mature enough to go to authorities and report the situation, so that the law can begin advocating on her behalf.
Having access to emergency contraception as a teen doesn't eliminate the negate circumstances at home. The abuse has to be addressed head on. If knows to purchase a pill...she knows to go to a guidance counselor at school, a teacher or someone else.