The Gas mask Siblings are ghosts that appear in Season 4 part 2, Episode 1. Unlike the name, Gas mask Siblings are actually friends and are not related to each other.
When a fire breaks into a building, the siblings wore gas masks when their mom told them to. The flames were so strong that it broke the ceiling, killing the siblings, and turning them into Ghosts.
Each of them has their own unique ability. The oldest has fire, the girl can manipulate sound, and the younger boy can generate dust.