You guys keep mentioning that debuffs are good and all, but remember that this is a world where they don't know the importance of it, and they can perceive the difference, with that they seem useless to compare to a white mage which they could tell because it directly affects their body.
Well...
If the Siblings are willing to go this far, then it may have to be a connection to what our mc's father said after he left about his bloodline of his mother or something at the beginning of the series.
How is she smart enough to make gadgets and a powerful sword but so stupid by going "oh yeah these monster I have no chance against is definitely weak to my nimble ass jaws"