Movie-Specific : Contraband; Robert Wahlberg and Mark Wahlberg

[ayall]

I recently caught this flick (i was bored, and it was entertaining worth wasting a random sleepless week night), but i felt (and still feel) completely confused by the Robert Wahlberg v. Mark Wahlberg acting roles in this movie.

When I downloaded the movie, it said it was "Staring Robert Wahlberg" but the main character looked a lot like Mark Wahlberg.

I realize these guys are brothers, and imdb credits both of them;
http://www.imdb.com/title/tt1524137/

Anyway, I'm fairly certain the main character was Mark Wahlberg, but can't figure out where Robert Wahlberg is in the movie.

Is the Robert Wahlberg v. Mark Wahlberg some kinda alter-ego joke i'm unaware of?
(similar to the Richard Koufey v. Spike Jonze)