Your "cousin's neice by marriage" could be the daughter of your cousin's spouse's brother or sister, or the wife of your cousin's brother or sister's son, or even the wife of the son of your cousin's spouse's brother or sister. Obviously under the right circumstances your cousin's niece by marriage might actually be the same person as your granddaughter.
They may not be related at all. They have to share a common ancestor, it that does not appear to be the case here.
They do not share a common ancestor.
You and your grandmother's great niece are second cousins.
Your child and your niece are first cousins to each other. Your child and your niece's child are first cousins, once removed.
Your niece and one of your grandson's parents are first cousins. That makes your niece a first cousin once removed of your grandson.
Your niece and your son are second cousins. Your niece's child and your son are second cousins once removed.
Your brother's granddaughter is your great niece and you are her great uncle if you are a man, or her great aunt if you are a woman..
Your granddaughter and your niece's son are second cousins.
Your niece and your son or daughter are first cousins. That would make your niece and your grandchild first cousins once removed. Your niece's child and your grandchild are second cousins.
Your granddaughter and your niece's son (the grandson of one of your siblings) are second cousins to each other.
Your granddaughter and grand niece are second cousins to each other.
Your granddaughter and your grandniece are second cousins or third cousins to each other
They are first cousins once removed. Your niece's children and your grandchildren will then be 2nd cousins.
The granddaughter is the niece's first cousin once removed.
Your daughter and your niece are first cousins. Your daughter and your niece's daughter are first cousins, once removed.
Your 1st cousin's granddaughter would be your daughter's niece.
(1) Your niece is the daughter of your sibling (2) Your niece and your children are 1st cousins (3) Your grandchildren and your niece's children are 2nd cousins (4) Your g'children and your niece's g'children are 2nd cousins once removed
Your great great niece is the granddaughter of your niece or nephew. Your daughter and your niece or nephew are first cousins. Your daughter and your great great niece are first cousins, twice removed.
Your grandfather's first cousin's niece could be:your motheryour second cousinsomeone you are not related to (for a niece through marriage to spouse)