daytona and others.... As you know I'm only expecting my first so I don't have direct experience of adding a sibling to a family. However, with my work hat on (child
psychologist) I would say to try not to ascribe "older emotions" to toddler actions and reactions. As an adult, it's natural to feel guilty about adding a sibling and to therefore look at the change in your older child's behaviour and to think about experiencing jealousy, being left out, feeling rejected, feeling unhappy and so on. However, (especially for for 1-3 year olds) the majority of the unsettled behaviour will be a reaction to everything being different and them feeling unsettled and having to adjust to "the new normal". Hence sometimes more tantrums, clinginess, being overly attached to one parent, tearful, regression in toileting/sleep. Same as when they start nursery or for any big changes to home life.
Obviously, it's completely natural for the older child to experience some 'negative' emotions in response to a big change such as those that accompany a baby but most of the time they're just reacting on base instincts and it's us -the adults- who add the more complex emotions on top and make ourselves feel terrible by essentially putting our minds in their bodies.
I don't know if I've explained that very well. I suppose I'm saying to be reassured that it's not the baby they dislike, nor their parents.
My three year old niece completely blanked her little brother and parents for about 24 hours after they bought him home from hospital. It was just her way of processing the new changes, but it was very disconcerting whilst she was doing it!