Answer
The answer is, they were either first cousins once removed, or half-niece and half-uncle. Their genealogical relationship was due to Klara's mother Marie marrying Alois's father Johann Georg Hiedler, Johann being Alois's biological father. In 1876, Alois married his second cousin Maria Schicklgruber, whose paternal grandfather was Johann Nepomuk Hiedler, the biological brother of Alois's biological father Johann Georg Hiedler.