Francesca is 15 months. She already knows that's a baby inside me coz I always point at other babies on the street and point to my tummy. She got the idea. Everytime I ask her "where's baby Willow?", she points to my tummy and give baby Willow a kiss.
My mother in law told me how to avoid the older one 呷醋:
After you give birth and your older one come and see you and the baby, don't let them see you holding the baby in the first place. Give you older one a cuddle and show him/her the baby. Let them cuddle the baby if them wish.
When you get home, don't let them to see you carrying the baby. Make your husband to carry the baby. Give a cuddle + kiss to your older one and said you miss him/her.
The idea is: don't make them think the baby is the first piority.