Complex sentence: Denis had abandoned Ada and their two surviving children in New York in 1912, shortly which Ada and her daughters moved to Monrovia, California, where Ada could be treated at the Pottinger Sanitorium for tuberculosis.
Split into simpler sentences: Denis had abandoned Ada and their two surviving children in New York in 1912. Shortly after Denis left, Ada and her daughters moved to Monrovia, California where Ada could be treated at the Pottinger Sanitorium for tuberculosis.