It's important to let the preschooler know ahead of time what behavior is acceptable and what is not. You must also tell the child what the consequences will be if he disobeys. Once a preschooler knows the boundaries he must respect, he'll have a better grasp of what behavior is appropriate and what is not.
Once the child displays aggressive behavior, address it quickly. Young children may forget or block out bad behavior and won't understand what you are upset about if you respond too slowly. The quick turnover time will let the preschooler know exactly what behavior you are addressing.
Once you've set guidelines and consequences for aggressive behavior, you must follow through with the rules you have put forth. If you told your child he would get a time out for hitting a sibling, you must give him a timeout for hitting his sister. This will let the child know you mean what you say and help him rethink an urge to act aggressively.
Children remember rewards for good behavior, so it's important to use positive reinforcement. If your preschooler is acting kind or handles an argument without getting aggressive, make sure to tell him how proud you are. You may also want to give him a treat, like 15 extra minutes of playtime before he takes his nap.