If you only have 18-20 months or less (eek!!) between babies then jealousy may not be an issue. Children between the ages of two and four can react to the arrival of a new sibling in any number of different ways ranging from being totally disinterested, being extremely loving and gentle straight away, being over exuberant in their affections (or treating the baby like a play thing, trying to pick them up and carry them around whenever you turn your back), being very clingy and wanting to be babied again themselves, liking the baby but being very angry with Mum &/or Dad that things have changed right through to extreme jealousy and outright hatred of the interloper!!
Anecdotally, boys tend to be a little more jealous than girls. Try to remember that it is a phase which will pass.
I remember hating my little brother who arrived just after my fourth birthday. I have a very vivid memory of my sister and I staring at his bassinette in the corner of the living room and just wanting to do away with him! It didn’t last long though, we were a very happy threesome.
Here are a few ideas on helping your toddler/pre schooler adjust to his or her new sibling: