Great Uncle.
The great grandson of your great grandfather's brother is your third cousin
No, Brother-Sister relation
None. There is no relation through "my friend"
Your grandson and your brother's grandson are second cousins to each other.
It does not matter how long you have been married. Your wife's brother's son (his grandson) is her, and your, great nephew.
There is no relation between you and your grandmother's cousin-in-law's grandson.
Your grandson is your brother-in-law's great nephew, and your brother-in-law is your grandson's great uncle.
Your fathers brothers are your uncles. You are their nephew, or niece.
Your brother is a great great uncle to your great grandson. Your great grandson is your brother's great great nephew.
Your father's great grandson could be your grandson, or your great nephew.
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-in-law is your spouse's brother. His grandson s your spouce's great nephew. (Some people say grand nephew). That makes him your great nephew, too. Your brother-in-law could also be the husband of your sister. Then your brother-in-law's grandson is your sister's grandson and your great nephew.