My niece was living with me and needs to pick up the rest of her stuff. Are there laws regarding how and when?

The police will not get involved cause they would see this as a civil matter. I would have the stuff ready and sent a message that you will be home at say at 7:00 and if they did not pick it up by 9:00 there would be one more chance and if was not gone by then, then to the dumpster it would go. She could then sue you in small claims, but you can also sue her for the back rent and any thing you are out in cleaning up the mess and for any damage she caused.