The problem is that it doesn’t just happen in movies.
It’s not at all uncommon for parents to decide that their children have different needs and should be treated differently in their wills. Perhaps one child has been significantly more successful than the other and the parents don’t believe that child needs as much help. Perhaps one child has borrowed a significant amount of money from the parents, and they feel that the other child should be equalized under the will by getting all of the estate, or at least a larger percentage of it.
Whatever the reason, if you want to leave an unequal distribution, it is always a good idea to talk about it with your children. If they are surprised, it can lead to hurt feelings and a breakdown in their relationship after you are gone. If they are angry, it is far better that they are angry at you while you can work it out rather than being angry with a sibling who had no control over the situation and can’t correct it.