Even thought you not related in genealogy sense you are related by marriage and you can say they are newphew and nieces in law.
The children of your husband's siblings are your nieces or nephews. Their children are your great-nieces or great-nephews.
your niece's or nephews
Their Names You Idiot
My brothers-in-law are my sisters' misters. I don't know which of the sisters' husbands he is.
cousins
Well, it depends how close you are to your husbands family. If you really want to know they are cousins-in-law.
spain
Yes, Cleopatra had two husbands, both of them were her brothers. She had four children with men who were not her husbands.
Your brother would call your husbands sister by her first name. Or, "my (sibling's) sister in law.
your second nephew
you call her your daughter's husband's sister
you call your parents sisters aunty