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General help:
jameda.de is Germany's leading web page for rating physicians.
The app of the Bundesärztekammer lets you search for physicians based on the languages they speak. As you usually cannot enter Med School without knowledge of English I suppose most physicians will state they are fluent in English and I suppose usually your sister will be fine.
You can search for physicians with the title Prof. (professor) such as scientific work in medicine in Germany is mostly in English their English is going to be way better.
Edit: Partially right, it was the Kassenärztliche Bundesvereinigung: https://play.google.com/store/apps/details?id=de.kbv.bas