The grandchild of your grandmother's first cousin is your third cousin. The grandchild of your grandmother's second cousin is your fourth cousin. The grandchild of your grandmother's third cousin is your fifth cousin.
Well that would depend on if you were the biological grandfather. If so then the grandchild is also your grandchild. If the child was born to a child of your ex-wife and not fathered by you then they are no relation to you and you would call them by their name.
Your great uncle's child is your parent's first cousin and his grandchild is your second cousin. His great grand child is your second cousin, once removed, and his great great grandchild is your second cousin, twice removed.
Your child and your sister's child share a common grandparent and are first cousins. Your grandchild and your sister's grandchild share a common great grandparent and are second cousins. There is a generation out of synch, so they would be second cousins once removed.
If your brother-in-law is the brother of your wife, then one of his sisters is your wife and any others are your sisters-in-law. If your brother-in-law is the husband of your sister, then his sisters are not related to you, but they are your sister's sisters-in-law.