Thank you everyone! I feel a lot better after reading through these. Leaving the house and taking my valuables with me sounds like a good solution.

To the people who asked, my niece sees a therapist and psychiatrist once a month (which should probably be upped to 2-3 times a month for the therapist), but I don't see a counselor of any kind. I'll looked into the Nami and their classes and the book Jasper and Mari mentioned. I don't think my niece has any emergency medications she could take. From my understanding, the only medications she has are Abilify and concerta. She's been diagnosed with things in the past- ADD, ADHD, ODD, and finally bipolar disorder.

I'm not sure if her tantrum counts as manic or not. She was fairly calm until she realized when knew she was lying, then everything escalated, and by the end she fell asleep on the couch.... Now that I've typed that out, it does sound fairly manic. I'll look into the states laws about leaving children alone, too. I live in Kansas, so our laws on that might be different.