No they are not brothers! Hell, they are not even remotely related! Undertaker's actual name is Mark Callaway while Kane's actual name is Glenn Jacobs. They are just storyline brothers.
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Kane and undertaker are brothers, and they've always hated each other. they've competed in inferno matches, burried alive matches, and casket matches. Kane has always seemed to have gotten rid of undertaker, but the undertaker always seems to "return from the dead". it's been a while since taker and Kane have been in the same ring. (as of may 2012) but don't expect that to last long ;)
Like a million times already, history keeps getting re-written over and over again.