Do you think the oldest child 's behavior (good and/or bad) influenced the way their parents treat their young siblings?
When you are looking at child development it is important to realize that a child's behavior is reactive to the environment they are in. What this means is that a parent shapes the child's behavior through their actions, or lack of action, in response to the child's behavior. When you are looking at this concept you are looking at the child learning through classical and operant conditioning.
It is also important to realize that parents also learn through classical and operant conditioning. If the parent reacts to a behavior of an oldest child and receives a positive reaction (e.g. the behavior stops or continues as desired) the parent is more likely to repeat this behavior with subsequent children. If a parent repeats a behavior that they utilized with an older child and the desired outcome is not obtained extinction will occur and the behavior of that parent will stop and will need to be replaced by a new behavior.