Legally, your girlfriend needs to get her underaged little ass back to her aunt's house where it belongs. The legal age for a minor to move out is not 17 - it's 18.
Because of an anomoly in your state's law, law enforcement in Missouri is sometimes reluctant to force a 17 year old back home if they are in a safe place. However, sometimes does not mean always and there is nothing in the law that would prevent her aunt from coming to get her herself. Nor is there anything in the law that would protect you and your family from having charges filed against you and possibly finding yourselves in jail if you do not send her home when you know her parent/legal guardian wants her there. So the answer to your question is an unqualified NO. When she is 18 she can live wherever she wants. As long as she is 17, if her legal guardian says she goes home, then legally she goes home.