Complex sentence: Visiting his client at her hotel, he learns her real name is Brigid O'Shaughnessy, she never had a sister, and Thursby was an acquaintance who had betrayed her.
Split into simpler sentences: Visiting his client at her hotel, he learns her real name is Brigid O'Shaughnessy. She has no sister, and Thursby was an acquaintance who had betrayed her.