MIX barbecue sauce, 2 Tbsp. lemon juice, curry, cumin and pepper; pour half over chicken in shallow glass dish. Stir. Refrigerate 20 min. to marinate.
HEAT grill to medium heat. Remove chicken from marinade; discard marinade. Thread chicken onto 6 skewers alternately with onions. Grill 8 to 10 min. or until chicken is done, turning occasionally and brushing with remaining barbecue sauce mixture.
MEANWHILE, combine remaining lemon juice, sour cream, cucumbers, mint and garlic. Serve with kabobs.