Lately my sweet little boys aren't always sweet with each other. This is totally normal, I know. In part this occurs in our life because DS2 is at home alone with me all day while DS1 is at school. When DS1 comes home, my attention is divided - not popular for either one! DS1 wants to visit and talk about his day, while DS2 still wants the attention he had all day.
There's another factor at work here. Research shows that friendships are relatively easy to maintain, in part because the friend goes home. A sibling can't do that. You know what it's like to have a best friend and think you'll live together GREAT, but it turns out to be a fiasco. This is because time away from our friends actually helps our relationships. Siblings don't have that luxury.
This isn't necessarily a bad thing. Eventually they may want to spend their life with some special someone and skills for living day in and day out with the same people are hard earned. And siblings can share a special bond throughout their entire lives.
What's a parent to do? A few things. First, be sure they have time apart. Don't always force them to play together. Respect the boundaries that they set up. Kids aren't always suave at boundary setting, though! My DS2 will often say "I don't want you to read a book to me!" (you can hear the shouting whine, right?) What he's really saying is that he doesn't want DS1 with him right then....or doing that activity with him.
So time apart is healthy. So is teaching them communication skills for living with others. Skills like you'll learn at Your Successful Family!
Finally, and I know you know this but it can't be overstated: be sure to tell each one what is special and lovable about that specific child. Spend time doing what that child enjoys and protect that time as much as possible. This is a delicate balance, especially as the number of children increases. But with effort, it's workable, and worth it!