B1
a daughter of your brother or sister, or a daughter of your husband's or wife's brother or sister
Dictionary examples:
My niece asked if Santa Claus was my brother because we're both chubby. (0.0)
My niece asked if she could grow money on trees in our garden. (0.0)
My niece tried to trade her baby brother for a puppy at the park. (0.0)
We're taking my niece on vacation with us. (21.3)