I think you are
the United States of America, looking at its borders.
my grandfather's other son would be my uncle, so we're looking at Uncle Sam.
I look toward the eagle, thorn, and snake
And find I may someone from danger take
Eagle, thorn, and snake describes the coat of arms of Mexico, which is South of the US. To take someone from danger is to SAVE them.
When facing Atlas' rough and salty brine,
Beyond the wall is where I now incline
The first line clearly refers to the Atlantic Ocean, which is East of the US. Inclining beyond the wall, one might find an EAVE.
The cellulose destroyers next I see.
I split a house of worship centrally.
Cellulose destroyers could refer to beavers, found in Canada (thanks @GarethMcCaughan), which is North of the US. Splitting the centre of a church is a NAVE.
At last I view the place of peace and rest.
A gesture of farewell describes me best.
The Pacific Ocean sounds like a pacific place, a place of peace, which is West of the US. A gesture of farewell is a WAVE.
A greeting in a land that's ceased to be:
That suits my circumstances pointlessly.
AVE is a greeting in the dead language of Latin, and this is the (compass-)"pointless" version of the words SAVE, EAVE, NAVE, WAVE.