Second cousin.
Great Uncle.
nothing
Your sister's son is her grandson and your great nephew. Some use the term "grand nephew."
Your father's sister is your aunt. Her son is your first cousin. Her grandson is your first cousin, once removed.
There is no relation between you and your grandmother's cousin-in-law's grandson.
Your sister's son is her grandson and your great nephew. Some use the term "grand nephew."
Your father's great grandson could be your grandson, or your great nephew.
Your grandson is your sister's great nephew and she is his great aunt.
You and your uncle's grandson are first cousins once removed.
You grandson is great great nephew to your uncle. You uncle is great great uncle to your grandson
You grandson is great great nephew to your uncle. You uncle is great great uncle to your grandson
The grandchildren of siblings are second cousins to each other.