"When a brother or sister is 1.5 - 3 years older".
At an early age, when the older child is 1.5-3 years old, and the youngest was only born, you can notice changes in jealousy. And at such a moment the older brothers are more subject than the older sisters. The kid is jealous of his mother to her little sister or brother, not understanding why there is so much attention not to him, but his mother rarely does with him and only when the youngest child sleeps. In such a situation, you need to give it as easier to understand that such a time situation is temporary and very soon a sister or a brother will begin to walk and then children will be able to recognize each other and play on an equal footing.