Your grandchild and your niece are first cousins, once removed.
You are great great aunt to your niece's grandchild. Your children and your niece's grandchild are first cousins, twice removed.
If you have a niece or nephew, it is their daughter.
Great grand niece
They are cousins.
Great niece or nephew.
Your great grandfather's great niece is the grandchild of his brother or sister. That makes her the second cousin of your mother or your father, and therefore your second cousin, once removed.
Your sister's great grandchild is your great great niece or great great nephew.
Yes, of course your son and your great niece are related. They are first cousins, once removed, to each other. (Your great niece's parent, your niece or nephew, and your son are first cousins to each other.)
A person's grandchild and grand niece are 2nd cousins to each other.
They would be your step-grandchild.
Your nephew's child is your great nephew or great niece depending upon sex.Your nephew's grandchild is your great great nephew or great great niece, depending on sex.
Great nephew. Your children are the first cousins of your niece and your children would be first cousins once removed. Your grandchildren and your niece's child would be second cousins.