Parents and the elder son

Parents are there to love all the children equally. Thus, parents will be embracing all of them.
In order to support parents, the elder son is there to help his younger siblings. Thus, what parents would do and what the elder son would do will not be the same. Even though they might share the same heart, how they would express it may be different.

In other words, God's expectation upon the parents and God's expectation upon the elder son is not the same. So, we cannot say, "If they share the same heart, why do they act different?" For example, parents cannot scold or chastise his child in a certain way that the child would leave the family. Elder son representing parents has the role to educate his younger siblings even through scolding and chastising, which parents could not do. Thus, sometimes the elder son could not be as nice as their parents to his younger siblings. He will grow by doing that role.