This is based on a situation early in the novel The Witness by Norah Roberts.
Suppose that a sixteen-year-old girl (E) witnessed a serious crime, in fact a double murder, apparently by an organized crime figure. E reports the crime to the police, gives a detailed statement, and identifies pictures of those involved from photo-spreads. The people she identifies are known to the police as active members of an organized crime group.
At this point E's mother (S) arrives at the place where E is being interviewed. S, who is the sole custodial parent of E, demands that E come home with her, and that she not go off with the US Marshalls to a safe house, and not prepare to testify.
S is well-off, and can afford to hire private security, and a good lawyer.
This all happens in the city of Chicago (or more exactly, one of its neighborhoods, Lincoln Park) in the state of Illinois, in the US.
Can S legally insist on E coming with her, over the objections of both E and the US Marshals and police? Can S obtain a court order requiring the Marshalls to surrender custody of E to S?
Update I should have mentioned that the police and representatives of the US Marshal's service believe, and state to S, that E is in serious danger from the organized crime group involved.