It's a fact of life: Siblings squabble. Ways to help make peace now and set them up for friendship later:
Don't play the blame game. Toddlers and preschoolers lack the problem-solving skills to settle spats, so they'll need your help (and patience) to learn to share and take turns. Focus on what needs to be done now to defuse the trouble, whether it's separating the kids or removing a toy, rather than on who did what or how it started -- after all, it takes two to tangle!
Make each kid feel secure. Siblings often fight for your attention, so help them feel confident in your unconditional love, no matter what their behavior or feelings. Instead of scolding your child ("Don't say that about your sister!"), show that you accept her emotions: "Sounds like you're pretty upset with Sarah right now." On PopSci.com: Why parents are more strict with older kids
Encourage sibling revelry. Compliment your kids whenever they show empathy or consideration for one another: "It was very sweet of you when you gave your little brother a hug."