Combine all marinade ingredients into a gallon ziplock bag. Cut sirloin into 1½ inch cubes. Place sirloin cubes into marinade bag. Remove excess air. Make sure all the beef is covered in the marinade. Place ziplock bag in the refrigerator for 8 hours or overnight.
Cut vegetables into 1½ inch chunks. Drain marinade from ziplock bag. Assemble kabobs by alternating vegetables and steak. There should be enough to make 8 kabobs with 2-3 pieces of meat per kabob.
Heat grill to 400-450℉. Spray kabobs with avocado oil <or drizzle oil> before placing on grill. Add a little salt & pepper to taste, if desired. Grill kabobs for 12-15 minutes, turning 2-4 times while cooking. Keep the grill covered while cooking.
Use a meat thermometer to check for doneness. Cook to 135℉ for medium-rare, 145℉ for medium, 150℉ for medium-well. Serve with rice or pasta and more veggies, like corn on the cob or a green salad.