Hola Richard
Antes que is comparing an action between Mercedes y Enrique and her/his father, between two or more nouns For instance: lávate las manos antes que tu hermano
Antes de indicates one action goes before another one. For instance: lávate las manos antes de comer.
Anyway you can find as well antes de que, and there is a kind of argument if that is correct or not, but it is at the moment, and many people say it. In this case it comes with subjutive, Come algo antes de que te vayas.
I am sure there will be a good explanation from a teacher soon.