What to do about sibling bullying?

[5ChildrenAndIt]

... Or at least persistent meanness, belittling and criticism from my pre-teen DD to her 6yo brother.

Come down really, really, really hard on the bully. It's utterly unacceptable.

I had the same problem with DC1 10yo bullying DC3 7yo (and to a lesser extent DC2).

We have a zero tolerance policy and things have definitely improved. I try and praise her every time she says/does anything nice.

I do try - but I fear that punishment is feeding the bad atmosphere between them.

Same as isshouldhave
DC1 is clearly bullying DC2 ... No sure what method is the best especially for all the low level bullying...
I have yet to find a book that addresses this issue.

The problem is that I think it is driven by frustration rather than malice. DS really can be quite hyper/distractable.

I keep pointing out to DD that if she was more polite and constructive in saying stop jumping/singing/walking about in a daze/spilling your food on me - she would probably have a point - but in the moment DD often picks an 'over assertive' strategy to express her frustration.

Looking from the outside in, the effect is basically of a bouncing puppy getting kicked. We somehow need to change the pattern for both their sakes and boost their bond and their communication.