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).
It would be her cousin
Your sisters daughter would be your niece
She would be a niece
Great niece
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.
Your younger brother's wife's sister's daughter would be your son's cousin.
Grandaughter.
neice