(as) sure as death and taxes

(as) sure as death and taxes

Certain to happen; inevitable and unavoidable. A variation of the proverbial phrase, "Nothing is certain but death and taxes. They just aren't right for each other; they're sure as death and taxes to break up eventually. The two brothers will be forever at each other's throats, as sure as death and taxes.
See also: and, death, sure, taxes