Since the budgie that's being attacked is the one that might be sick, it's best to keep them in separate cages for now if you can. If she actually is sick, the stress of being attacked isn't good for her.
It's possible that these attacks are related to breeding hormones. It's spring, and birds usually consider the cage to be their territory and a potential nest area, and maybe Kaylee wants the cage to herself because of this. There are hormone control techniques that can help reduce breeding urges, but it takes a week or so for it to kick in:
http://www.littlefeatheredbuddies.com/i ... mones.html