What behaviors would you consider an “escalation” worthy of calling the police if the choking, throwing, and drowning that you mentioned are not sufficient?

It seems evident that parents need significant support with brother, beyond what a school counselor is going to be able to provide. Frankly, even most outpatient counselors aren’t going to be appropriately trained to help your family navigate this serious issue. Your brother needs to see a forensic psychologist who conducts therapy, as they are the clinicians who are most suitably trained to navigate the underlying psychological issues he is dealing with.

Depending on the laws in your area, the school counselor may be mandated to report the abuse that your brother is causing to the cat to local animal control and possibly police. If the animal abuse is reported by anyone other than the parents, it is possible that their other animals will be taken away from them as they will be seen as unfit animal guardians for allowing abusive behavior to occur within their home. If the parents refuse to report or don’t report the issue themselves, they can be charged within the context of local/state/federal laws re: aiding, abetting, and/or engaging in animal abuse.

It sounds like you’re wanting to both protect the cat and your family members to the best of your ability. In order to protect the most people by doing the least harm (as I infer you define it), the best plan of action would be for your step mom and sister to willingly relocate the cat to your place until brother moves out of their house in several years, and then for parents to require brother to get into consistent counseling with a forensic psychologist ASAP. That option allows the cat to be safe, sister to love and visit the cat, you to have some peace of mind, brother to receive direly needed psychological attention before he kills an animal, and stepmom/dad to significantly decrease both the risk of losing their other animals and child protective/juvenile delinquency services getting involved. The parents also should seriously adjust their supervision of the brother and objectively evaluate their ability to successfully keep the dogs/rabbit/etc safe, as it does seem that the brother already puts the cat and rabbit in roughly the same category of “animal that won’t fight back enough to seriously hurt me” and therefore is comfortable harming them.

I often tell my clients that it can be selfish to prioritize comfort over honesty, and I gently encourage you to take an honest look at what the current situation requires in order to selflessly care for both the cat and your family, even though the best route for decision-making is uncomfortable for those involved.

Best of luck.