Your great aunt's grandchildren are your second cousins because you share a great grandparent. Your great aunt's great grandchildren are your second cousins, once removed, because you share the same common ancestor but are one generation differently removed from that ancestor. You are the third generation from that common ancestor, while they are the fourth generation.
Your great aunt's grandchildren are your Second Cousins.
You are related "by blood" to anyone who is descended from someone you are also descended from. That includes your great grandfather and great grandmother and all their children, grandchildren and great grandchildren.
Your great grandparents' brothers and sisters are your great great aunts and uncles.
Irene Houston Williams and Patricia Houston Williams were great grandchildren of Sam Houston
Your aunts and uncles are the sisters and brothers of your parents. So they are "blood related." But you also call the husband of your mother's sister your uncle, and he is not "blood related." So aunts and uncles are not always really related to you.
Your great grandparents brothers and sisters are your great uncles and great aunts.
The grandchildren would be the parents, aunts and uncles, and first cousins once removed, of the great grandchildren.
Your third cousins are the great grandchildren of your great great aunts and great great uncles.
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.
The grandchildren of any of your great uncles and great aunts are your second cousins.
yes
great nephew
Your grandchildren and your great niece are second cousins to each other.
great great (etc.) grandparents great grandparents grandparents + great uncles and great aunts parents + uncles and aunts [=parents' siblings] siblings + cousins [=uncles' and aunts' children] children + nephews and nieces [=siblings' children] grandchildren great gandchildren great great (etc.) grandhildren also cousins and second cousins [=(great-)grandparents' descendants]
great great (etc.) grandparents great grandparents grandparents + great uncles and great aunts parents + uncles and aunts [=parents' siblings] siblings + cousins [=uncles' and aunts' children] children + nephews and nieces [=siblings' children] grandchildren great gandchildren great great (etc.) grandhildren also cousins and second cousins [=(great-)grandparents' descendants]
The children of your uncle are your first cousins. The children of your first cousins are you first cousins once removed. Thus the grandchildren of your uncle are first cousins once removed. The related link I will make below explains how all this works.
Your children are the parents of some of your grandchildren. They are also the aunts or uncles of those of your grandchildren who are not their children. Even you have a child who is younger than one of your grandchildren, that child is still the aunt or uncle of all those grandchildren.
Your grandfather's brother's grandson is your great uncle's grandson. The grandchildren of great aunts and great uncles are your second cousins.