Complex sentence: Her daughter Irina Nijinska (1913-1991), a dancer in her own right, had helped her with her work for many years, both at her ballet school and as her rehearsal assistant.
Split into simpler sentences: Her daughter Irina Nijinska (1913-1991) was a dancer in her own right. She had helped her with her work for many years, both at her ballet school and as her rehearsal assistant.