The peony parrot fight!

Question
I got two peony parrot from my friend, and they are sibling, their relationship were always very good and intimate for 8 month already.
    
     However, few days ago, I found out the older one was attacking the younger one in a very brutal way......and due to the older one is much stronger than the younger one, therefore, the younger parrot was very damaged and seemed very sacred, the younger parrot even refused to return into the cage eversince(they live together).
     
     I dont understand what happened to them, and how can i reverse thier bad relationship back to those good old time? I really dont want to see any one of these two gets hurt anymore!!!!(especially the weaker one!!)

Answer
-- Unlike humans brother and sister; brother and brother; sister and sister or even parent and offspring birds are not meant to stay together beyond their weaning into independance.  

What you're seeing is perfectly natural.  What you're doing by trying to keep them together is actually cruel to them and is making them aggressive, stressed and eventually may kill one of them.

If you want to keep them both you need to have two cages.  BIG cages.   Each bird needs to be able to stand on the center of their perch and fully outstretch their wings, while completely turning around and not touching any part of the cage with a single feather.

That's a cage that's big enough.

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For more on bird care properly, http://www.4animalcare.org/birds