My only sibling is a sister 2 years younger than I am. We are EXTREMELY different. Growing up, we fought a lot. As in all the time. We did get along for short periods of time (like when we were making fun of our parents, LOL), and sometimes we did even voluntarily do things together, but for the most part, we didn't get along.
Now that we're adults, we get along much better. It's been improving ever since college, and having kids has really helped, since we do a lot of things similarly. There are some childrearing issues that we don't agree about, but we're both really big breastfeeding advocates, so when we start to argue, we just work the topic back to breastfeeding so that we can stop arguing.
Sorry, this probably isn't what you wanted to hear. But we are really good friends now, and I think that if my parents had been more concerned from the beginning, things could have been better. Because of my own childhood, I'm very careful to limit my children's exposure to books and TV that depict siblings fighting or that emphasizes the idea that older children don't like playing with younger children. Our culture really reinforces those ideas, and I know that when I was growing up, I thought that I was
supposed to dislike my sister. I want to keep that idea from being implanted in my kids' heads.