The daughter of my father's brother?

• I think you mean "what do you call my father's sister's daughter?"
• She would be your 1st cousin and your fathers niece.

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• Niece by definition is the daughter of one's brother, sister, brother-in-law, or sister-in-law. While
• Cousin is by definition the son/daughter of your uncle or aunt.
• Accordingly, the daughter of the sister of your father is your cousin
• Refer to link below