It is always awe-inspiring to me when I see siblings helping each other or cooperating during play. Sometimes this comes naturally to kids and sometimes it doesn't. Although their genetic makeup and personality play a part, parental behavior and guidance are big factors in this equation.
As we all know, kids learn from example and if you, as a parent, are teaching, through your own relationships with your partner and your own relatives, values such as kindness, compassion and patience, these things will rub off on your children. It is not always going to be peace, quiet and happiness with your kids but your example will be helping to build their set of values and character.
Another way you can help your kids to develop their sibling relationships is to take charge of their activities. I'm not referring to being a dictator here but I do think all parents need to take an active interest in their children's daily activites. Kids who spend a large majority of their time, unsupervised, watching television or playing games on a computer are not interacting with their siblings. Some activites along these lines are great but parents need to make sure that there are other activites that require cooperation and participation. Games, in particular, are good since children have to learn to get along and display a certain amount of patience with each other. These can be indoor or outdoor games, for example, send them on scavenger hunts as a "team" rather than independently.
Plan family outings whenever possible. Encourage the kids to help each other to make lunch together for a picnic outside or build a tent outside. If you go to the park or beach, let your children know that it is a "family" outing and that, as a family, everyone is responsible for helping each other so they will be ready to go on time. Little things that seem so inconsequential can make large contributions to relationship building. There is nothing better than having a great relationship with a brother or sister as you grow up and become an adult.