Prepare the marinade: in a medium sized bowl mix the soy sauce, banana ketchup, chili sauce, lemon juice, Sprite soda, garlic, sesame oil, salt, black pepper. Do not add sugar yet. Blend ingredients well. Pour over the boneless chicken thighs.
Seal bag and refrigerate overnight.
When ready to cook, soak 8 bamboo skewers in water for 30 minutes. Thread one thigh on each bamboo skewer. Reserve marinade.
Pour marinade into a saucepan and add brown sugar. Bring to a simmer over medium heat. Set aside to use as basting sauce. (if not comfortable reheating the marinade and chicken juices, make a new batch for basting).
Pre-heat the grill to a medium high for at least 5 to 10 minutes. Grill the chicken pieces for a total time of 20 to 25 minutes, basting every now and then to have a shiny glaze. Flip the chicken pieces after the first half of cooking so they cook thoroughly on all sides.