When the family grows, parents can help to build the relationship between siblings.
For example, you can help your older child to choose a present to give the baby in the hospital and prepare another present that the new baby will give to the older sibling. This is a good way to introduce the children to each other.
Then, when you arrive home with the new baby, you can ask your older child to help you to take care of the baby.
If your older child is still a baby, you can suggest him or her easy things as give the baby the pacifier, sing a song or something similar.
When the older sibling is aged 3 or more, you can let him or her hold the baby on his or her lap, always under your supervision.
If the big child is having problems to adjust him or herself to the new member of the family, perhaps is easier asking your child to help you directly instead of forcing him or her to interact with the baby.