Providing alchohol and drugs to a minor is a crime.
You can tell the police what is going on. An extreme measure which will have ramifications, but under the circumstances may be indicated. If you have proof, the adults in the family could be arrested, particularly if the police catch them in the act.
Penalty is fine up to $2,500, jail of 6 months. First offense is probably few hundred dollars, suspended sentence and a misdemeanor record, not to mention being in the paper.
If she is considered a runaway, police can bring her back. Also, housing a runaway is a crime. If she has permission to go it is different.
You might be better off dealing with the other family directly. Of she goes without your consent, the police can bring her back. They would also get a chance to look around this other family's house. That is why I would go after the alcohol and drugs aspect. That will get the police's attention.
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Can she serve them with anything informing them that they are not allowed to let her stay there? They are not cooperative with my sister.
She can write to them and state that you have not given any permission to her daughter to stay at the house. That might come in handly later.
The idea is to put them on notice, in no uncertain terms, that the daughter is violating your sister's wishes. She is just 13, that is a baby. They have exposure on alcohol, drugs, and harboring a runaway.
If she has time, your sister might want to stop by the station and have a chat with someone.
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