What can I do to help my children become better friends?
First of all, there is no way to make someone like someone else. Some sibling relationships will be stronger than others. But you can create an environment that helps your children to feel positive about each other. Also keep in mind that children may not show affection the way adults would. Think about the indications you see that the kids don't like each other. Are you sure they really don't like each other, or are they enjoying picking on each other? And think about their relationships at other ages. If they have had a close relationship during earlier time periods, there is probably a base that still exists and will appear again at some point.
Children often will be more supportive of each other when they are away from the family. They sometimes can be very protective of each other. So it may be that they demonstrate a more positive relationship when you are not around. Give them ways to share responsibility for each other when appropriate. Encourage them to watch out for each other. And make sure they know that you love all of them individually and together.
Siblings learn a lot about relationships by dealing with each other. Studies have shown that children who have positive relationships with their siblings are likely to have strong friendships, too. But children with the weakest friendships outside the family are those who have no relationship at all with siblings. Even sibling relationships that include a lot of conflict are teaching children some basic relationship skills.
To help the sibling relationship become more positive, make sure you are not setting up competition between your children. Let them know that you will be as fair as possible, but that fair does not mean that the treatment will be equal. Help the children understand each other. Spend some time now and then during family meals or family meetings with everyone saying something special (and positive!) about the other people in the family.