I am a us citizen, i would like my British niece to come be my nanny for a year. Is that possible?

Theoretically, it is possible. You could use an H-2B visa to petition for a nanny, but you will need to demonstrate no United States citizen or lawful permanent resident is available and qualified for the job. You will likely also face difficulty establishing the job offer is bona fide given the family relationship. You can read more about H-2B visas at http://myattorneyusa.com/h2b-visas.