Dear Michael,
This is very unusual. Most of he time siblings will play fight and even roughly but they should not have so much hostility and aggression. It is possible that they did not receive enough food at their prior home and learned to fight for everything. The best way to remedy this is make sure they are overfed. Leave PLENTY of food in their bowels so they will become confident it will always be available. Also get feliway plugins at a pet supply of www.catfaeries.com as that will safely keep them calm.
PLease let me know how this works out and if you have more questions. I will be happy to help.
Very sincerely,
Terri
Dedicated to Cats and the people who love them!
Please press ACCEPT if my answer was helpful and leave feedback Thank you! Terri