Great grandniece or great grandnephew
Great aunt
The common ancestor is your parents. Your children and your sister's children are first cousins. Your grandchildren and your sister's grandchildren are second cousins. Your great-grandchildren and your sister's great-grandchildren are third cousins.
No, they`d be the second sisters great nieces and nephews
Your sister is the great-aunt (also written great aunt) of your granddaughter.
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 borther-in-law is your grandchildren's great uncle. They are his great nieces and great nephews.
Your grandfather's sister is your great aunt. The grandchildren of your great aunt are your second cousins. Their children, the great grandchildren of your great aunt, are your second cousins, once removed.
Your grandfather's sister is your great aunt. The grandchildren of your great aunt are your second cousins. Their children, the great grandchildren of your great aunt, are your second cousins, once removed.
your great grandchildren
They would be cousins to some degree. They are 2nd cousins.
Yes, he is their great uncle.