Cousins
You are their aunt or aunt by marriage.
Your brother's daughter is your niece..and you are her AUNT.
a relationship between brothers should be sacred and good....
Great Niece.
YES
Your daughter's daughter is your granddaughter, and you would be her grandmother or grandfather.
2nd cousin
She is your first cousin.
Your daughter is your brother's niece. Your brother is your daughter's uncle
Your nephew and your daughter are first cousins.
That would be her great uncle I suppose. Common sense.
First cousins twice removed.