aunt
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).
they would be second cousins
daughter-in-law
Your grandchildren and your grandniece are second cousins to each other..
your great grandchildren
Great aunt
Your grandchildren and your niece are first cousins, once removed, to each other.
The name of the relationship between your grandchildren and your sibling's grandchildren is called being "second cousins." The genetic relationship between two families becomes smaller and smaller through each generation, but no matter how weak that relationship is, it's still counted as a blood relationship. Your grandchildren and your sibling's grandchildren share a pair of great grandparents.
One of the twin sons of Judah by Tamar (Genesis 46:12)(Numbers 26:20)(Ruth 4:12)
Your borther-in-law is your grandchildren's great uncle. They are his great nieces and great nephews.
They would be cousins to some degree. They are 2nd cousins.