Your sister's daughter is your niece and you are her aunt (if you are a woman) or her uncle (if you are a man).
Your brother's daughter is your niece..and you are her AUNT.
Great Niece.
YES
You are their aunt or aunt by marriage.
She is your cousin
That would be her great uncle I suppose. Common sense.
First cousins twice removed.
Your daughter's step-sister would be your step-daughter.
Your aunt's daughter is your first cousin.
neice
Grandaughter.
Your younger brother's wife's sister's daughter would be your son's cousin.