First cousins three times removed.
If you are a woman, you are your great grandmother's great granddaughter so your son is the son of her great granddaughter. But there are other possible relationships to other of her great granddaughters.The great granddaughter could be your sister. Then your son is her nephew and she is his aunt.The great granddaughter could be your first cousin. Then she and your son are first cousins, once removed.The great granddaughter could be your second cousin. Then she and your son are second cousins, once removed.
They would be the fourth cousins. They share a common 5 Great Grandparent.
You and the granddaughter of your great grandmother's first cousin are third cousins, once removed.
Your grandson and your sister's grandson are second cousins to each other.
If you are a girl, your are your great aunt's cousin's granddaughter. If you are a boy, your sister is. Your great aunt also has first cousins from whom you are not descended. For those first cousins, the granddaughter is your first cousin, twice removed, or your first cousin, or your second cousin once removed, or your third cousin.
If your great great uncle (the brother of your great grandparent) had a great great granddaughter, you would be third cousins, once removed.
If you are a girl, your are your great aunt's cousin's granddaughter. If you are a boy, your sister is. Your great aunt also has first cousins from whom you are not descended. For those first cousins, the granddaughter is your first cousin, twice removed, or your first cousin, or your second cousin once removed, or your third cousin.
An uncle's nephew and the uncle's child are first cousins to each other. The uncle's great granddaughter and the nephew are first cousins twice removed to each other.
Your daughter and your niece are first cousins. Your daughter and your niece's daughter are first cousins, once removed.
Your granddaughter and your grandniece are second cousins or third cousins to each other
Your child and your nephew are first cousins. Your granddaughter and your great nephew are second cousins. Your granddaughter and your great great nephew are 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.