Complex sentence: Lulu promised the Baudelaires that she wouldn't tell Count Olaf where they were if they took her to the Mortaim Mountains but she didn't keep her promise and Count Olaf captured the Baudelaires.
Split into simpler sentences: Lulu promised the Baudelaires that she wouldn't tell Count Olaf where they were if they took her to the Mortaim Mountains. However, she did not keep her promise and Count Olaf captured the Baudelaires.