that's only legal in Alabama
That answer makes no sense. It is obviously legal for an aunt and uncle to have sex. I think the correct thing to do is this: Whoever feels more comfortable with these sorts of things, the aunt or the uncle, should talk to the niece. Since she is 16, she should be fine but a bit embarrassed. Apologize that she saw that, and say that it's awkward for all of you right now but the awkwardness will pass. Ask her if she has any questions, she'll probably say no, and that will be the end of it.
Your niece will call you her aunt or uncle.
You are their uncle (if you are male) or aunt (female).
Your sister's daughter is your niece. You are her aunt if you are female, or her uncle if you are male.
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.
If your a boy they would call you great uncle, if your a girl they would call you a great aunt.
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.