I understand that the eldest son must (or pay someone to) complete the incomplete salaah after the parents death – please correct me if I'm wrong.

Therefore, if the duty is towards the son after the death of his parents, is there any shar'ee duty during the lifetime?
The son may be regarded as the oldest son but may have an older sister i.e. The son is 30 years old and his sister is 35 years old?

by

The eldest son is responsible to makeup the prayers/ fasts missed by the father by mistake, or performed wrongly, due to ignorance, not the one missed intentionally.
The eldest daughter is not responsible to do so.