Ruth Fischer and the Eisler brothers
As other list comrades have pointed out, Fischer was the sister of the Eislers.
Lou is unfair in calling her a Trotskyist. The particular ultra-left tendency that she and her boyfriend Arkadi Maslow led in the German Communist Party was associated with Zinoviev's faction in the Comintern, part of the Stalin-Zinoviev-Kamenev 'Troika' in the Soviet Union, and was very hostile to Trotsky. Maslow and Fischer's faction took hold of the KPD after the Brandler-Thalheimer leadership was pushed out following the abortive attempt at power in 1923. It seems that Trotsky had no sympathising faction in Germany during this period. That developed later.
Fischer and Maslow had a short and abortive period in the Trotskyist movement in the 1930s. They went into exile during the Second World War, as left-wing Jews they would have been immediate targets of the Nazis, and ended up in Cuba, where Maslow was killed in a motor accident. Fischer then went to the USA. Her book Stalin and German Communism is an odd work, not least because of the large number of errors which various bourgeois and left-wing scholars have pointed out, and the fact that it is self-serving -- attempting from a right-wing social democratic stance in the late 1940s to justify the ultra-left policies she and Maslow carried out in the early 1920s.
Fischer has always appeared to me as a loud-mouthed exhibitionist, lacking any political judgement. Clara Zetkin, who knew her quite well, said that the orientation of her politics depended upon the outlook of the man with whom she was romantically related at any particular point, although this may just be a nasty aside from someone who detested Fischer both politically and personally. It would, however, be worth finding out the political orientation of the bloke (I think he was French) she married after Maslow's death to see if Zetkin's statement had some validity. It seems that she was by no means an ultra-left before she met Maslow.