This picture makes it seem like my 19-month-old twin boys get along great and for the most part they do....
...however when they don't, I've often thought about how to handle this as a parent. I realize that EVERY brother and sister fight...but when does the normal "fighting" turn into something more that requires serious attention? I recently came across this, which helped me:
The problem comes about when sibling rivalry takes on a tone of bullying...this sort of bullying is not normal...(read more here) from the Health News Digest
There is certainly a fine line and for every parent that line is a little different.
One thing my wife and I have done is give EXTRA praise to our kids every time they share something. A great example of this is after bath every night, while my wife gets one of the boys in their PJ's...the other one will go grab 2 pacifiers that we leave out and go give it to the other boy. (It's actually really cute to watch - they will put it directly in the other one's mouth) and when they do my wife and I always say "gooood boy!" and clap.
Since we clap every time they do that, now they clap when they do it and get really excited. Again, it's pretty cute and makes us feel like we might be doing at-least one thing right in this whole parenting thing!!