I notice the sibling has smoother feathers, so I guess it'd be a safer bet to suppose it's either a deficiency that affects keratin, or a disease or inherited disorder which does the same.
On this forum I've seen a few people with chicks that look the same and it's not something that improves with time, from what I've seen, but I have yet to find out what it actually is.
Hopefully someone who does know can come and tell us. All the best.