The daughter of my father's brother?

Answer 1

• 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

Answer 2

One's sister's daughter, or one's brother's dayghter all called niece.

Answer 3

The daughter of your father's brother (of your uncle) is your first cousin .

Answer 4

If it is your biological father's daughters, they are your sisters, (siblings).
If it is your step-father, they they are your step-sisters.