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Third cousins once removed.
If the grandfathers were 1st cousins, the grandchildren of each grandfather are 3rd cousins to each other.
Your grandfather's sister's children are your 'first cousins once removed'.
The relationship to the grandfather or the relationship to the grandson of the grandfather? Let's say your name is Ken, your grandfather's name is William, his brother's name is Eli, and Eli's son is named John. If you're asking about William and John's relationship, they are first cousins. If you're asking about Ken and John's relationship, they are first cousins once-removed.
3rd cousins
If your great grandfather is her great uncle, then you and she are second cousins, once removed.
Your grandfather's niece is your parent's first cousin. Her daughter is your second cousins and her granddaughter is your second cousin, once removed.
You could be 7th cousins 6 times removed or 9th cousins 4 times removed.
Your great uncle is the brother of your grandmother or grandfather. Therefore you and your great uncle's grandson share one or two great grandparents, and you are both the same number of generations separated from that shared ancestor. So you are second cousins to each other.
If two people have the same grandfather, they are siblings or first cousins. If two people have the same great grandfather, they are siblings, first cousins, or second cousins to each other. If two people have the same great great grandfather, they are siblings, first cousins, second cousins, or third cousins.
If your great grandfather's were brothers, then you are third cousins.
Two people whose first common ancestor is their great grandfather are second cousins. That describes her and your parent. Since you are one generation further from the common ancestor than she is, the two of you are second cousins, once removed.