When does this happen? When are you away from them for long periods when
this could be happening? Does it happen at night? I think you should do some
detective work. Are the two siblings at odds or is one a bully?

I suggest playing with all of your cats every single day for at least an
hour each day (2 30minute sessions, 4 15minute sessions, whatever you can
do). It is possible that the young cats are trying to expend energy through
rough and tumble play and it's gone too far. The aggressor (the one doing
the biting) seems to have acquired a bad habit that needs to be stopped.
When you play with the cat anytime any cat is aggressive with another one
for no apparent reason, correct the aggressor. Say "no" and separate the
cats. Be persistent, be calm, don't shout, and praise your cats for good
behavior. Make sure you only discipline a cat at the moment they are
misbehaving so they associate your admonishment with what they are doing at
that moment. Then make sure you praise them for minding you. Positive
reinforcement actually does more good than the negative. The "no" stops them
by startling them slightly (don't yell it scares them) and the positive
praise causes them to learn to continue to stop what they are doing when you
say the word "no" to them. Use a firm, commanding tone, not harsh and do not
use physically intimidating body language such as leaning down toward the
cat. You don't want them afraid of you, you want them to behave. Only use
your body language if they don't mind your "no" command.  Eventually, the
aggressive cat will learn that biting is not okay.

If this is occurring at night, or you don't know when it's happening, keep
the victim near you as much as you can. When one of my cats was using the
other as a chew toy at night, I would bring the one being nibbled on into my
room at night. I had to have a separate litter box, food and water bowl
available for her so she could be in with me for 8 hours every night. I also
played with the cats more during the day to squelch the offender's pent up
energy. I only had to do that for a few days and she is no longer bitten at
night. My cats are older, though. Since yours are younger, it may take a
little longer before the biting stops.

I advise you finding out more about who is doing the biting for sure and
when it happens, if you can. The more information you have the quicker and
easier the solution will be. Good luck!

For more advice, talk to your vet, surf the web for pages on cat behavior
and solutions.