Q: If the eldest child of a deceased person is a female and his/her second child is male, is it obligatory for this son to perform the missed prayers and fasts of the mother and the father?
A: The criterion is that the male child is the eldest among the sons, if the father has any sons. As for the assumption in question, it is obligatory for the son, i.e. the father’s second child, to perform his father’s missed prayers and fasts. By obligatory caution the same rule is applied to his mother's missed prayers and fasts as well.