Your aunt's grandson is the son of your First Cousin and is therefore your First Cousin once removed
Ref: http://freepages.genealogy.rootsweb.ancestry.com/~gentutor/chart.html
Your niece and one of your grandson's parents are first cousins. That makes your niece a first cousin once removed of your grandson.
If you really need a relationship term, use granddaughter-in-law. Most people would merely say "my grandson's wife."
The grandson of your nephew is your great great nephew, but few people use that relationship as a form of address. Usually they call relatives by name or business title, not by relationship.
Your great-grandmother's sister's grandson is your second cousin once removed.
Kublai Khan was Gengis Khan's grandson
second cousin
Your aunt's great grandson is your first cousin twice removed.
Your son and your aunt's child are first cousins, so your son and your aunts grandson (the son of your aunt's child) are first cousins, once removed.
Your sisters grandson would be your grandnephew.
it's like your 3rd cousin
Your grandfather's brother's grandson is your great uncle's grandson. The grandchildren of great aunts and great uncles are your second cousins.
Your brother is your grandson's great uncle, and your grandson is your borther's great nephew. Some people would also say grand nephew and grand uncle.
Trey Songz has several aunts, as he comes from a large family. Specifically, he has mentioned having multiple aunts in interviews, but the exact number isn't publicly detailed. Family dynamics can vary widely, and since he has a close relationship with his relatives, he often references them in his music and personal stories.
Your niece's child and your grandson's child (your great grandchild) are second cousins, once removed, to each other.
Your grandson and your brother's grandson are second cousins to each other.
Your niece's grandson is are your great-grandnephew.
They are your Uncles and or Aunts.