It seems unfair, but it's a father's right to leave his inheritance as he wishes. Perhaps he knows his children well enough to know which ones would handle money more responsibly. Maybe he knows that giving one child a huge amount of money would be like giving liquor to an alcoholic .In the long run, it would end up destroying them. Or, maybe one child has children so they have more obligations.
When we get money as a gift or any gift, why do we have to compare it to what someone else gets? Why can't we just be happy and content with the gift we receive?