You might also mention to her that it seems like her frustration actually came from a GOOD place, not a bad place. She was frustrated because she was watching another creature suffer and there was nothing she could do about it. That is one of the most painful things to endure, for anybody. It speaks of her empathy and compassion. That she felt bad about it afterwards just shows how incredibly sensitive she is. I went through a similar thing when my mother was dying and my attempts to comfort her kept failing ... I got so frustrated I reacted with anger at her, and then later felt like an awful, terrible human being. But she herself understood why I was reacting with anger, and other family members had nearly identical reactions. When I told a therapist about it all later, she told me exactly what I'm telling you here -- frustration is a completely normal reaction to feeling helpless in the face of another's suffering. And anger is too, as crappy as that is. The most important thing here is that your sister was aware of her emotions and actually felt bad about it. If anything, that means she's not abusive.