We can't all get on all of the time - it's natural for your children to argue and fall out with each other. The relationship teenagers have with their siblings can help to map out the way they will socialise and deal with disagreements when they grow up.
One of the reasons brothers and sisters find it hard to get on, is because of differences in age that mean they are at different stages in their development and are interested in activities relating to that.
Introduce activities and days out that all age groups can participate in, such as a trip to the park - younger children can play on swings whilst the older ones may want to kick a ball about. This means that no one is getting left behind and there's no cause for resentment.
Encourage the older sibling to help their younger brother or sister, such as with a game or by building something together. This can make the older child feel important and a special part of their younger siblings' life.
Sometimes younger children feel jealous that their elder brother or sister is allowed to do things that they aren't - such as having a later bedtime or staying out with their friends for longer. Explain that they will be able to enjoy the same privileges as they get older. Point out the advantages of being young - such as they don't have to help around the house as much as their older brother or sister does.