Hi Mel,
Behavior is always a challenging subject to deal with. When you have sibling cats suddenly fighting, I would first look at considering if they are resource guarding – they should have their own separate food bowls, a multitude of toys, and you should have at least 3 litterboxes in the house. Next, I would consider if there are stresses in the home or other changes – visitors, death or birth in the family, job changes that affect the schedule, use of air fresheners or plug-ins. It wouldn’t hurt to get a vet check up for both cats to make sure one or the other hasn’t developed a condition that the aggressor might be picking up on or experiencing.
I hope this helps.
Dr. Ryan Llera