Watch for the Signs
If you hear your eldest child belittling their sibling's achievements, making comments like: 'Anyone could do that!', or 'You're such a know-all' or 'You think you're better than everyone else.' You'll know that they feel that they are losing their position of 'top dog'.
You may notice that they avoid participating in any activity where they can be directly compared with their brothers and sisters, in case they don't measure up.
What's the Problem?
The first step is to understand what lies behind the competitive behaviour. Listen to the grievances - they will reveal the origin of their feelings.
Older children will have grown accustomed to outshining their younger siblings at everything, but as children get older, the difference in their ages becomes less pronounced and the younger ones appear to catch up. It is not surprising if they get frustrated when their younger brother or sister comes up with the right answer before they do.
They feel threatened and challenged!