This is a book I read in the 70s or maybe early 80s. I believe it was set in England or somewhere not in the US. It was about a young girl who had been diagnosed with diabetes. Her parents bought her a horse. She has a mean sister and I remember at Easter she got all this sugar free stuff and her sister got a big chocolate egg and she lunged for it and her sister ended up busting her head open. That’s all I remember. I hope you can help!