Great-great-great Aunt/Uncle. You basically aren't listed as anything else after the first 'great' to that line.
Your borther-in-law is your grandchildren's great uncle. They are his great nieces and great nephews.
You would be their great-granduncle or great grand aunt, and they are your great-great nieces or great-great nephews.
No, they`d be the second sisters great nieces and nephews
They are your great nephews and/or great nieces. Alternately, some people say grand-nephews, grand-nieces.
Your aunt's grandchildren would be your first cousins once removed, as the your aunt's children are your first cousins.
Your grandfather's brothers would be the uncles of your parent. That would make them your great uncle.
You brother is your grandchildren's great uncle or granduncle.Your grandchildren are your brother's grandnieces and grandnephews
The relationship term is usually hyphenated "great-grandchildren" (grandchildren's kids).
Your mother's sister's children are your 1st cousins.
Second cousins.
your great grandchildren
Great aunt