Fold a 36x18-inch piece of heavy foil in half crosswise. Place sweet peppers and onion in the center of the foil. Drizzle with oil; sprinkle with 1/2 teaspoon of the fajita seasoning and the garlic. Bring up the opposite edges of the foil; seal with a double fold. Fold in remaining edges, leaving space for steam to build. Set aside.
Sprinkle the remaining 1 teaspoon fajita seasoning on both sides of steak; rub in with your fingers. For a charcoal grill, place steak and the vegetable packet on the rack of an uncovered grill directly over medium coals. Grill steak until desired doneness, turning once halfway through grilling. Allow 14 to 18 minutes for medium-rare doneness (145°F) or 18 to 22 minutes for medium doneness (160°F). Remove steak and keep warm. Grill vegetables about 20 minutes or until tender, turning packet once halfway through grilling. (For a gas grill, preheat grill. Reduce heat to medium. Place steak and vegetable packet on grill rack over heat. Cover and grill as directed.)
Meanwhile, wrap tortillas in foil. Place tortilla packet next to steak on grill rack; grill about 10 minutes or until tortillas are heated through, turning packet once halfway through grilling. Thinly slice meat into bite-size strips. Divide meat among tortillas; top with vegetables. Roll up. If desired, serve with salsa and sour cream.