Combine marinade ingredients in small bowl. Place beef steaks and 1/3 cup marinade in food-safe plastic bag; turn steaks to coat. Close bag securely and marinate in refrigerator 30 minutes to 2 hours. Stir brown sugar and salt into remaining marinade; set aside.
Remove steaks from marinade; discard marinade. Place steaks on grid over medium, ash-covered coals; arrange bell peppers around steaks. Grill steaks, covered, 7 to 11 minutes (over medium heat on preheated gas grill, 8 to 13 minutes) for medium rare (145°F) to medium (160°F) doneness, turning occasionally. Grill peppers, 9 to 12 minutes (for gas, 7 to 11 minutes) or until tender, turning occasionally.
Remove steaks; let stand 5 minutes. Carve steaks across the grain into thin slices; season with salt and black pepper, as desired. Cut bell peppers 1/2-inch strips. Serve beef with bell peppers. Drizzle with reserved marinade; sprinkle with olives. Garnish with cilantro, if desired.
