The children of your father and the children of your father's first cousin are second cousins to each other.
The children of your father and the children of your father's second cousin are third cousins to each other.
You are each others cousins.
First cousins twice removed.
They are your cousins.
You and your first cousin's children are first cousins once removed. Your children are their second cousins.
Your children are the first cousins of your brother's children, and the first cousins once removed of your brother's grandchildren.
You and your second cousin's children are second cousins, once removed. Your children and your second cousin's children are third cousins.
Your child and your sister's children are first cousins. Your grandchild and your sister's children are first cousins once removed.
Your children and your nephew's children are frst cousins, once removed
Anyone descended from a brother or sister of one of your direct ancestors is your cousin. There are different names for different kinds of cousins. For example, your first cousins are the children of your aunts and uncles. Your second cousins are the grandchildren of your great aunts and great uncles. Your parents'first cousins are your first cousins, once removed. And the children of your first cousins are also your first cousins, once removed.
Your sister's children's children are her grandchildren.Your children and your sister's grand children are first cousins, once removed.Your grandchildren and your sister's grandchildren are second cousins.
Your granddaughter and your first cousin's grandson are third cousins to each other. You find their common ancestor that is, your grand parents, and count the number of generations separating them, there are Brother and Sister then cousins, you and your cousin would be one (first cousins), your children and your cousins children are second cousins and their children are third cousins and so on.
If you have two or more first cousins who each have children, those children are second cousins to each other. If you have two or more second cousins who each have children, those children are third cousins to each other. And so forth. It is more complicated is you are looking at cousins who are removed one or more times. And you should remember that you have cousins who are not related to each other, so their children are also not related to each other. This would be the case, for example, if one cousin is the child of your mother's brother and the other is the child of your father's sister.