Ask your son and DIL to leave (taking sister) if they fail to abide by your wishes. Son or no son he has to respect your wishes in your home.
My answer is not intended to be giving legal advice and this topic can be a complex area where the advice of a licensed attorney in your State should be obtained. Please click "helpful" or "best answer" if my answer added any value or add a "comment" if you have more info for me to help you get a better answer.
You can contact the police and inform them of the situation. In some cases, the police will come out and ask her to leave. If she doesn't leave she can be arrested for trespass. There are probably other, civil remedies, but a 17 year old should leave if the police come out
No legal advice is given here. My responses to questions on Avvo are never intended as legal advice and must NOT be relied upon as if they were legal advice. I give legal advice ONLY in the course of a formal attorney-client relationship. Exchange of information through Avvo's Questions & Answers forum does not establish an attorney-client relationship with me. That relationship is established only by joint execution of a written agreement for legal services. I am only licensed in the States of California and New York and the District of Columbia
I suppose that you could initiate eviction proceedings or seek assistance from CPS.
The above is not intended as legal advice. The response does not constitute the creation of an attorney client relationship as this forum does not provide for a confidential communication.
You have a tough situation. She is technically not a tenant since everyone concedes she is a guest, but the longer she stays the more "tenant" rights she has. Same with your son and daughter-in-law. Hopefully she will in fact go back next week. If not you can try calling the police but it is highly unlikely the will do anything and you will probably be forced to go to landlord tenant court to get her out.
I know that this is not the answer you are looking for, but my advice is to do your best to resolve the situation without involving the police or any lawyers. I have two reasons for saying this:
First, even if you two are like oil & water, the fact remains that she IS your daughter-in-law's sister. I would hate for you to create a family rift because of something that blows up to be bigger than it needs to be.
Second, the girl is a minor and a run-in with the police or the legal system is the type of thing that could negatively impact her at a critical developmental period in her life. (How many of us at that age were perfectly gracious guests or had a clue what responsibility meant?)
Now, with that said, if the problem is more severe than you have indicated in your post, and there is a threat of violence, or drugs, or other serious variables involved, then by all means call the police and inform them that she is trespassing.
But if at all possible, try to resolve the situation without involving the law.