I think a parent should not differentiate between children. Ill-treatment of any child will affect him badly.
Blessed and privileged are those elder siblings who shouldered responsibility and sacrifice for the sake of the younger siblings as well as for the whole family and who are looked upon with regard and affection and taken care of by the younger siblings.
It is for this reason that they say the marriage between the eldest form each family will not be too happy. As they have shouldered responsibility at their childhood times, they expect some pampering from their spouses, which many times may not happen to come due to sheer realities of life and they again slog on as responsible adults.
But marriage between the eldest of one family and youngest of the other family is said to work well. As they get what they expect and imagine opposite roles of their childhood roles as pampering and pampered.
Knowledge is power.
small drops make a mighty ocean
In middle-class families, elder children have to be more responsible and see that their siblings will come up in life. Otherwise, it will be a difficult situation for the parents. I have seen in some families, where the elder brother struggled to bring their siblings up in their lives but never received any support or appreciation from his siblings.
My father is the eldest son of the family and in his 18th year, his father passed away. He discontinued his studies and settled for a small job so that he can take care of his two younger brothers and mother. But his brothers even after settling well in their lives never helped him. He managed alone all the way till his children start supporting him.