Your half-sibling's grandchildren will be your half-great neices or half-great nephews, and you will be their half-great aunt or half-great uncle. Confused Permalink Submitted by Anonymous on August 20, My father's brother married my mother's sister; what is the genetic relationship between me and my uncle and aunt's daughter, compared with the children of the sister of my father and uncle? And yet, according to the test, we share only 0. My maternal great grandmother and my paternal great grandmother was the same person. They would be half cousins I think. So if I have a child and a subsequent grandchild, and my brother has a grandchild, what are the grandchildren to each other? His brother and sister are married to another set of sister and brother.