No they are your great nephew or great niece and you are there great aunt or great uncle
Yes. your sibling's sibling's child. Yes. 2 siblings. Basically your niece's first cousin is ur niece/nephew as well
Second Cousin
Your cousin's child is your first cousin, once removed. Then when that child grows up and has a child, it will be your first cousin, twice removed. Now, if you have a child, and your cousin has a child, your children will be second cousins to each other. Then their children will be third cousins to each other.
Your niece is the daughter of your brother or sister.Your first cousin is the child of your parent's brother or sister.Your second cousin is the grandchild of your grandparent's brother or sister.Your third cousin is the great grandchild of your great grandparent's brother or sister.
Your daughter and your niece are first cousins. Your daughter than your niece's son are first cousin's once removed. Your daughter's child and your niece's child are second cousins to each other.
Your grandfather's niece is a first cousin of one of your parents. Her son is your second cousin.
Cousins
Your child is your cousin's second cousin. (And he/she is your child's second cousin)
Your first cousin's child is your child's second cousin. Your second cousin's child is your child's third cousin. Your third cousin's child is your child's fourth cousin. and so forth
Your 1st cousin's child is your 1st cousin once removed. Your 2nd cousin's child is your 2nd cousin once removed. And so on...
Your father's first cousin's child is your second cousin. Your father's second cousin's child is your third cousin. Your father's third cousin's child is your fourth cousin. and so on.
Your child would be the second cousin to your cousin.