Rema in Shulchan Aruch, 470:2 says that a father should fast for his first born (or attend a siyum) on Erev Pesach, if the son is too young to fast, himself. He also states, that if the father, himself is a first born, then the mother should fast for the son.
Let's say the father died, or for some reason, the father cannot fast for health reasons. Is the mother obligated to fast or attend the siyum for her son?