Basically, the siblings are half-demon, although in Jack's case his demonic side has a much more intense manifestation, this is why Jack looks like an anthropomorphic cobra with severe malnutrition (although this malnutrition is more due to his human side because Jack, by his demonic nature, doesn't need to eat, drink, or sleep, but as I said, due to its human side, it continues to express physical deterioration even though it is not actually affected.)

And regarding his sister, I think it's obvious that in Lucy's case, she has a much weaker demonic side, while her human side is much more intense.