thanks a lot guys, it came very surprising. We'll keep her spirit alive, but it's very hard to loose someone so close to your heart. That's why I need to find this book, preferably the german version. I found a hardcover, which I remembered to be in my possession once in my childhood. I think the story could be part of it...
https://www.amazon.de/Turtles-Die-He.../dp/3785523637