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The police may make a mandated report to the state's child protection agency since she is a minor and could be at risk of harm. However, there is no guarantee anything will be done. Is the 12 year old home by himself?
If the town has a curfew for kids you can get the police involved. Also if they are drinking or up to no good the police can do something. If they are just walking around with no curfew they can just say they are walking to a friends house and nothing can be done. But if it happens again, ring the police and make up a story, like your elderly mother just rang you because a group of teens were outside her house screaming and yelling, she only got a glimpse of one girl (give description) You cant remember your mothers house number. They will (if they're not busy) go for a drive around looking for said girl and friends, and call parents as they are disturbing the peace.
Its not illegal, no. Its not ok but not illegal. Theres no law that states what time a 14 year old needs to be home, its up to the parents discretion on what is or isnt appropriate. Im not sure that you can do much since she isnt in your care. Im not sure the police would be able to do much, her mums not home so how can they make her go home or keep her there.