You are doing the best you can during this stressful situation. eWhat helped me with my DS3 is re-framing the situation and view of the child's intentions in a more positive, empathetic light as a problem to solve together instead of as manipulation and bad behavior. The former deepens your bond and is calming to your child, and the latter just makes both parties angrier and more frustratd.
It sounds like DD6.5 fears and anxieties are so INFLAMED by DD3's diagnosis that a pinprick really does feel like a punch right now. Accepting that all her hurts are real to her may greatly change the dynamic. Of course, you will still discipline her by instructing her on acceptable behavior, but removing the parent's negative emotional reaction from the moment, verbal or non-verbal, is the challenge.
Help her develop coping skills and regain control over fears that are very valid to her by building mutually agreed upon positive strategies for regulating her emotions. One example is the book, "How Does Your Engine Run? A Leader's Guide to the Alert Program for Self-Regulation" Shellenberger, S. Although written for children with sensory dysfunctions, I think the toolkit can be modified to fit other situations. For example, when she hurts herself, she may be seeking attention or affection, so include positive, alternative options for seeking attention in her tool-kit. Another excellent book for developing collaborative problem-solving strategies is "The Explosive Child" Greene, R.
Give her recognition for the positive strategies she has already tried - learning about diabetes and helping her sister with healthy eating habits.
A quick tip that works for me is I tell my son that I'll talk about it when he's calm. He know he's heard, but I won't engage with him on the topic until the meltdown is over.
Right now all your emotional resources must feel stretched to the limit. I wish you all the best.
This was my instinct. Wishing the OP the best.
