Complex sentence: Her ability to speak Russian was an asset given to her character Maggie DuBois, justifying the character's recording the progress of the race across Siberia, and entering the race at the beginning as a contestant.
Split into simpler sentences: Her ability to speak Russian was an asset given to her character Maggie DuBois. It justified the character's recording the progress of the race across Siberia, and entering the race at the beginning as a contestant.