Teach your toddler the meaning of 'share'...which means your toddler will need to share mommy with the little one. Include your toddler on as much of the preparation for the arrival forward.
My two are 22 months apart (now 19 & 21) and my now ex late mother-in-law asked if she could keep our daughter at camp, on the lake, so I could relax a bit before taking care of two little ones.
At the time it was a great idea...however during those couple of weeks she brainwashed our daughter; basically she would no longer be in the center of attention because of her baby brother, etc.
The first time our daughter saw her little brother she said 'take it back' and for a while totally resented his presence.
All of my positive work was undone within a couple of weeks.
So with that said...listen to what others may tell your toddler, they may think it is funny or cute but it makes a big impact on the little one.