This could be
a) your aunt on your mother's side (i.e. your mother's sister)
b) a wife of an uncle who is your father's brother (an aunt by marriage).
You would be their great Aunt.
That would be your cousin so no you can't.
The relationship to you would be your cousin !
Your sister would be her great aunt and she would be your sisters great niece.
your second nephew
They would be great nieces or great nephews.
What family members your is fathers mothers daughters sons sisters
He is your cousin
Your first cousin
Your cousin.
Great Uncle.
Most likely they would not be related to you