Complex sentence: It is most unfortunate that all of Alma's biographers have accepted this unfounded "story" without questioning her preposterous allegation, because it is a disgusting and shameful lie which has now been thoroughly disproved (see the corresponding Wikipedia article on Emil Schindler).
Split into simpler sentences: It is most unfortunate that all Alma's biographers have accepted her unfounded "story" without questioning her preposterous allegation. This is however a disgusting and shameful lie, and has now been thoroughly disproved (cf. the corresponding Wikipedia article on Emil Schindler).