How your preschooler adjusts to having a new sibling will depend largely on his or her personality and temperament. Even children who are excited before your baby arrives may feel differently when they realise that your time is less devoted to them, however, generally more outgoing children will cope better and more sensitive children will need a little more coaxing to adjust.
You may find that your preschooler reacts by ignoring the baby, regressing to babyish behaviour or even aggression. All of these are normal, however, you will of course want to curb the aggressive behaviour.
Getting your preschooler to accept your new baby is a matter of getting him or her involved. Give your child special, age appropriate jobs to help you with the new baby, or spend time reading stories about families. Talk to your child about the baby, or spend time together watching when the baby sleeps.