Complex sentence: The identity of her mother is unknown, but she seems to have been a profound influence on Speght's life, as her mother's death is referred to by Speght as an inspiration for her "Mortalities Memorandum".
Split into simpler sentences: The identity of her mother is unknown, but she seems to have been a profound influence. Speght refers to her mother's death as an inspiration for her "Mortalities Memorandum".