María Cardeña Espino

Taken from http://en.wikipedia.org/wiki/Guillermo_Kahlo

She died giving birth to their third child, he asked Antonio Calderón for his daughter Matilde's hand in marriage. After the marriage, Kahlo sent his and Maria's daughters away to be raised in a convent.

However, this states only 2 children : http://en.wikipedia.org/wiki/Frida_Kahlo#Childhood_and_family

Frida's parents were married shortly after the death of Guillermo's first wife during the birth of her second child...Guillermo and Matilde had four daughters, with Frida being the third. She had two older half sisters.

They married on 8/15/1893