One child cannot be punished, and the other should be praised — if the situation is serious, punish both at once. You also need to praise children together — invent general instructions for them or give them easy tasks in the house — the more they will be, the less they will have reasons for jealousy. Do not praise each of them separately — try to say as often as possible that, only by completing the task in the team, you can do it well. If you decide to encourage your children — it is better to select the same way for this — so you will also be able to avoid the appearance of a sense of jealousy between your children.