Are Ness and Lucas from Super Smash Bros Brawl brothers?

No - Ness and Lucas are unrelated. They come from the MOTHER series of Video Games.

Ness is in the 2nd Mother Game, which is called Mother 2, a.k.a. EarthBound.

Lucas is in the 3rd Mother Game, which is called Mother 3.

Lucas lives hundreds of years after Ness, and they don't even know each other exist. They kind of "came together" in Brawl, though.