I am German and spend the first 35 years of my life in Germany and so I believe I do understand how children are seen and raised in Germany (obviously a generalization but I have found this true for most German families I know)
With this background here the books that helped me most to overcome (or at least question and work on) my German influenced view of parent - child relationships:

Unconditional parenting
The myth of the spoiled child 
Punished by rewards - all by Alfie Kohn
Playful parenting - by Lawrence D. Cohen

None of these are books about unschooling but wonderful jumping boards for a parent - child relationship to become a more respectful, loving and peaceful partnership. 
In regard to your sister in Germany it might be advantageous that these books are not specifically about unschooling. Homeschooling - let alone unschooling - are concepts that most Germans will disregard as "crazy" right away. At least that's my experience. 



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Saturday, April 16, 2016, 11:01 -0700 from [email protected] <[email protected]>:

I often wish there was radical unschooling material describing the parent/child relationship outside of a homeschooling context. We benefit so much from the trust we exercise since we started applying unschooling principles. But I only found out about this way to parent after considering homeschooling. Also, my sister in Germany has a son who suffers physically from going to school. School is mandatory there. I'd love a book about how to establish a relationship of respect and trust based on unschooling principles even when your child goes to school. Is there such? Sandra, do you feel there is need and would you write one please?