No matter how close or far apart you chose to have children, they will at one time or another experience jealousy towards their siblings. It's human nature, and not a result of poor parenting, so don't doubt your instincts on raising your children. Sibling rivalry can bring on a very stressful time in the life of a young family. However, children have to be taught by example when they are young, for it is not instinctive at birth that they know how to love and be kind toward one another.
Conversation can be very helpful as a start.
Talk openly with your child when you first discover you are pregnant. Talk about what is happening in the family unit, why mommy's belly is getting so big or why there is new furniture coming into the house, maybe even into their room. Take them along to prenatal doctor visits and let them hear the baby's heartbeat. Include them on simple decision making, like deciding between two names for baby or minor decorating decisions in the nursery. When a new baby is about to enter the family, it raises serious concerns within a child of any age.