sisters in law
My brothers-in-law are my sisters' misters. I don't know which of the sisters' husbands he is.
The children of your husband's siblings are your nieces or nephews. Their children are your great-nieces or great-nephews.
Well, it depends how close you are to your husbands family. If you really want to know they are cousins-in-law.
spain
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
They are sisters in law.
It is simply not true that a majority of the wives and their husbands' sisters don't get along and are mostly at loggerheads. Some women are like that whether they are sister-in-law or not. It has more to do with personalities and culture.
She is your niece.
Well, I call her Abigail, but different strokes, etc.