Your children and your husband's niece are first cousins. There is a difference of one generation between them. That makes them first cousins once removed.
your niece
Your granddaughter and grand niece are second cousins to each other.
Great niece
You granddaughter is a great great niece of your aunt. Your aunt is your granddaughter's great great aunt.
You are the great-great-aunt/uncle to your niece's granddaughter. This means that your niece's granddaughter is your great-great-niece. The term "great-great" signifies that there are two generations between you and your niece's granddaughter.
Your sister's daughter is your niece. Your sister's granddaughter is your great niece Your sister's granddaughter's daughter (your sister's great granddaughter) is your great great niece. You are her great great aunt, if you are a woman, or her great great uncle, if you are a man.
Your granddaughter and your grandniece are second cousins or third cousins to each other
that would be your niece or if you want to be fussy then your niece in law!
Your brothers granddaughter is your grandniece.
Your great great niece is the granddaughter of your niece or nephew. Your daughter and your niece or nephew are first cousins. Your daughter and your great great niece are first cousins, twice removed.
Technnically, she is not related as you do not share a common ancestor. She would refer to you as Aunt and she is your niece. Whether he is your first, second or fifth husband, his niece is your niece while you are married to him.
Your granddaughter and your niece's son are second cousins.