Springtrap and Silhouette are closer than any siblings could ever be. 

They were so close that Sil even stood up for Springtrap when Zepher hurt him

Then....Zepher couldnt take Silhouette, and sent him to Military School, where Sil became a soldier....and spent half his life in Afghanistan.

When sil saw his brother for the first time in 10 years..he cried