Complex sentence: She chose as her base a derelict building at Bildschön or Bielyczny near Kulmsee in the area controlled by the Teutonic Knights, whose Grand Master, Anno von Sangershausen, was a relative of hers, and the knights sheltered her.
Split into simpler sentences: She chose as her base a broken down building near Kulmsee in the area controlled by the Teutonic Knights. The grand master of the knights was a relative of hers, and the knights sheltered her.