You'll no doubt have noticed that parents of only children become their child's chief companion, especially for the first half of their childhood. If you have more than one child, you can step back and encourage them to play together. Of course, you can step in sometimes for a treat (to yourself) when they're tired and crotchety, or one of them really wants to play alone. The aim is that on balance your children grow up seeing one another as their primary companion. That sort of bond lasts right through their lives.
Encourage Them to Support Each Other
When your child asks for help with their homework, or getting the DVD player to work, or mending a broken toy, there's no need to do it yourself every time. It's far better for your children to grow up regarding it as normal to help each other out. Of course there'll be resentment if you forever make one of them drop what he's doing in order to sort out something for his brother and sister. But as long as you don't overuse this tactic, it's very effective. Your other child will be flattered if you say, "I'm hopeless with technology, but I bet Tilly can help you," or "Math isn't really my thing. Try asking Charlie."
It's great if you can find ways in which younger siblings can help older ones. Anything that irons out the hierarchy is a good thing, because once they're grown up it won't be helpful. They'll want to see each other as equals. Okay, so your younger child might struggle with his older sister's math homework, but he can still fetch a pair of socks from her wardrobe on the way downstairs, or help look for her escaped gerbil.