Complex sentence: While they were not always on the best of terms during school days (their personalities were much too similar), Nanny had been invited to Anne's 13th birthday party, and when they met in Bergen-Belsen, their reunion was enthusiastic.
Split into simpler sentences: While they were not always on the best of terms during school days (their personalities were much too similar), Nanny had been invited to Anne's 13th birthday party. When they met in Bergen-Belsen, their reunion was enthusiastic.