Certain kinds of threats are illegal, regardless of who makes them and to whom. "I'm cutting you out of the will" is a threat that an aunt might make to her niece that would, however, be perfectly legal. (She could make it to someone she's not related to also and it would still be legal, but it's not as credible a threat, since someone she's not related to would probably have no reason to suspect they would be in the will in the first place.)
It basically boils down to the fact that a credible threat (i.e. one that seems believable) of assault is in itself a type of assault.
"If you don't sit down I'm going to give you such a smack," though, is almost certainly both technically illegal and a good (and expensive) way to get a judge to laugh at the person who tried to pursue it in court.
Your niece will call you her aunt or uncle.
That's easy cousins.
Your Aunt's niece is you (if you are female), your sister, or your First Cousin.
You are your niece's aunt, if you are a woman.Any of your sisters who are not your niece's mother are also her aunts.The sisters of your niece's parent who is not your sibling are also her aunts.
You are her great-aunt/uncle. She is your great-niece.
You are an aunt/uncle-in-law! Congratulations!! You really are; because you're an aunt/uncle to your niece you are now an aunt/niece by marriage to her husband.
Your aunt's brother is either your father or your uncle. If you are a girl, you are always your aunt's niece.
Your aunt's son and her niece are first cousins to each other.
Your niece's son is your great nephew. You are his great aunt or great uncle.
The great aunt is the great niece's baby's great-great aunt.
Your sister's aunt (who is also your aunt) is the baby's great aunt. The baby is a great niece or great nephew of your sister's aunt.
You are your mother's aunt's great niece.