Here's what your older kid may be thinking: This baby can be fun, but he can also be a pain. He makes Mommy pay less attention to me, he cries and makes a mess and tries to play with all my toys! Since your toddler doesn't have the skills to express these sentiments calmly when he gets frustrated, yelling or acting out is often the most natural reaction. While all these feelings are totally normal when a new baby joins the family, there are ways you can help your son feel better about his younger sibling.
• Don't make every toy in the house community property. Your older child needs to have things that are just his. If the baby makes a grab for one of the "off-limits" toys, gently take it away and distract her with something else.
• Turn the baby's naptime into a special play time where he can play with his own big-boy toys, like Legos or puzzles, without the baby knocking things over or putting them in her mouth (which is a choking hazard, anyway).
• Empathize with your kid's feelings. Tell him you understand how frustrating it is when the baby cries or knocks over his blocks, but that this stage is only temporary.
• Make sure you spend quality alone time with your son so he doesn't feel like the new baby has to tag along everywhere.
All that said, at this age, your older child is big enough to understand behavior basics, so you should lay down some basic house ground rules. Explain gently, but firmly, that yelling at the new baby is not the way to get her to stop doing something or quiet down, and that each person in your house is expected to treat everyone else with respect. Remind him that as his younger sibling grows, the rules will apply to her as well.