Aggression towards siblings

Yesterday, our 7 year old daughter (diagnosed with ADHD) got frustrated with her younger brother (3 years old), and kicked him hard enough that he fell over. They were outside, so she was wearing shoes. Unfortunately this was not the first time that she has resorted to physical aggression with him. She also kicked him in the chest inside the house a couple of weeks ago.

The really disturbing thing that we’re seeing is that she doesn’t show any remorse for her actions and gets angry when we try to explain to her that what she did was wrong. At other times she yells, hits, or pinches. Also, on multiple occasions while having a tantrum she has screamed that she wants to kill us. We’re looking for any sort of advice or counsel on how to work through this issue. She’s such a thoughtful, deep, gentle girl and these moments are scary and we worry for her future (and the safety of our children) if she continues to lash out like this.