Yeah, they're "friends" off the field... like you and me are friends on Patriot's Planet.
I base it on observation, not conjecture.
The post game handshake in November, after Manning was upended again at the hands of Brady's Patriots was very telling. When you have just been bested by a "friend", who you probably haven't seen in a while, in a hard-fought game, I would expect the meeting to last more than 2 seconds, three tops! Maybe I'm odd, but if it were me in Manning's cleats, I'm quite certain that I would linger long enough to give TB his due. "Hey, great comeback, Tom....you got me again." and then a shared laugh or two and ending with a sincere "Good job....hope to see you again, in January"...or something to that effect. Just some indication, however small, that this "We are friends off the field" nonsense is nothing but PR on both their part, in all honesty.
Then there was the shot of Peyton in the luxury box during the 2007 Super Bowl, as the Giants were marching down the field for the winning score. There we saw Peyton, lurking as far back in the box as he could, to avoid being displayed to the world, I suspect, and reacting to the play on the field with obvious intensity. All things combined, it seemed to go way beyond rooting for your brother, rather seemed to be rooting against Tom Brady! After all, he knows another SB victory for TB and the debate is over!