Third cousin.
They are second cousins
Cousine
Your niece and your daughter are first cousins. Your daughter and the daughter of your niece are first Cousins, once removed.
They are second cousins
Your son and your nephew are first cousins to each other. Their daughters are second cousins to each other.
The children of siblings are first cousins, so your daughter and your brother's daughter are first cousins to each other.
They are first cousins.
third cousin once removed
Your daughter and your niece's daughter are first cousins, once removed.
Phoebe and Hepzibah were cousins. Hepzibah was the daughter of Phoebe's uncle.
Your son and your niece are first cousins. Her daughter and your son are first cousins, once removed.
Your niece and your daughter are first cousins to each other.