How do we make sure children of different mothers stay close as siblings when they grow up?

A bond built from childhood is the best foundation, but it has to be kept up with practice throughout life. Encourage them to keep in touch without waiting for big occasions — short messages, visits, and being there when one of them is in difficulty. Instil the value that siblings are who you lean on when your parents are gone. Avoid letting property become a cause of disputes; teach them to discuss openly and to refer to a proper authority if there is confusion over distribution. A good relationship is worth far more than any property. While you are still here, be the link that brings them closer, not a wall that separates them. See children in polygyny.