How do I add sibling to the top of a family tree that does not display parents? I am not able to add the brother of my great grandfather because data for his parents is unknown.

My search has stalled while researching great grandpa's parents. However, I do KNOW the name of my great grandpa's brother, but can't add him to the tree because available "relative" choices for great grandpa are only father/mother/wife/child. Brother or Sister (sibling) is not an option. DUH! --- Any ideas?