Does your family know kids who are big sisters or big brothers? Encourage them to talk with your child about what it's like to have a younger sibling. If you're an older sister yourself, share your own stories about growing up with a baby around.
Know what to expect by reading books like The Parents' Guide to Raising Twins by Elizabeth Friedrich and Cherry Rowland and Raising Twins by Eileen M. Pearlman and Jill Alison Ganon. Check bookstores and online for other titles.