Your grandson and your great great niece are second cousins, once removed, to each other
Your niece's grandson is are your great-grandnephew.
Your niece's son is your great-nephew. Your niece's grandson is your great-great nephew.
You are his great-great-aunt (or great-great-uncle) and he is your great-great-nephew.* A much less-used US term for the relationship is great-grandnephew which follows the logic that your niece's grandson is your sibling's great-grandson, so would have the same generation-separation from both you and your sibling.
Your great grandson and your husband's great niece are second cousins, once removed.
A great niece and a grandson of the same person are second cousins to each other.
great great nephew and u wud be a great great aunt... i think :)
Your niece's child and your grandson's child (your great grandchild) are second cousins, once removed, to each other.
Great great aunt/uncle. To your niece, you are the aunt/uncle, to her child you would be the great aunt/uncle. So you would then be the great great aunt/uncle of her child's child (her grandson)
Great nephew. Your children are the first cousins of your niece and your children would be first cousins once removed. Your grandchildren and your niece's child would be second cousins.
The son or daughter of one of your siblings children is your great niece or great nephew. In other words, the grandson or granddaughter of your brother or sister is your great niece or great nephew.
great great nephew and u wud be a great great aunt... i think :)
Your granddaughter and your niece's son (the grandson of one of your siblings) are second cousins to each other.