Brothers? Nah...
I bet in the original script, or at least the original idea, Delon and Palance's characters were meant to be ex-lovers. The way Palance refers to Delon "when we were together." And the ludicrous difference in accents, and then the way Palance flirts with the gay blond killer dude, playfully tweaking his nose and such.
Yep, many of the movie's problems could have been fixed by that one script adjustment: Palance is coming back to recruit his gorgeous ex-boyfriend back into a life of crime. It all makes sense...