Would your cousin's daughter be your niece?
The daughter of your first cousin is your first cousin, once removed. The daughter of your second cousin is your second cousin, once removed. First cousins share grandparents. Second cousins share a great-grandparent; but not grandparents.
Your niece is the daughter of your brother or sister. Brothers and sisters share parents.
To keep it simple, let's call your great grandmother's niece "The niece" The sequence of the relationship: (1) The niece is the daughter of your great grandmother's sibling (2) The niece's sister is also the daughter of your great grandmother's sibling, and therefore also her niece. (2) The niece and her siser are 1st cousins of one of your grandparents (3) The niece and her sister are 1st cousins, once removed, of one of your…
Your son and your grand niece are first cousins, once removed. Explanation: Find the common ancestor, in this case, your parents. Your parents have two children, you and your sister or brother. Your sister or brother has a son or daughter. This child would be your nephew or niece and your son's 1st cousin. A daughter of your nephew or niece is your grand-niece, thus your son's 1st cousin once removed. Note: Your grand-niece and…
Your grandmother's niece is the daughter of her brother or sister, thus the daughter of your parent's aunt of uncle and of your great aunt or great uncle. Your grandmother's niece's daughter is the granddaughter of one of her siblings. Grand children of siblings are second cousins to each other, so you and the daughter of your grandmother's niece are second cousins.
Either they are second cousins or technically not related at all. Your cousin's niece would be one of her siblings' children, in which case it would one of your cousin's daughters, or her spouses' niece, in which case there is no blood relationship, but most families would refer to them as a cousin.
Your niece is the daughter of one of you siblings. Your great grand niece is the great granddaughter of one of your siblings. If your niece and your great grand niece are both descended from the same one of your siblings, then they are grandmother and granddaughter to each other. If they are descended from different siblings, they are first cousins, twice removed, to each other.
A husband's niece is the daughter of his brother or sister. A wife's niece is the daughter of her brother or sister. In general, the two nieces are not related to each other. The only way the two nieces could be related would be if the husband and wife were related in some way in addition to their relationship through marriage, for example, if they were also cousins to each other.
There are a range of possibilities, depending on the exact relationship. If the niece is the daughter of one of his cousins (i.e., they are the child of one of the cousin's brothers or sisters) they would be a second cousin. If they are the child of one of the cousins' brother's or sister's spouse, there is no relationship, though most families would probably refer to them as a distant cousin.