It would be very helpful if the kids could be part of the preparations for the new baby in some way. Simply referring to him or her as "our new baby" is one way to help the children realize that the baby is not just going to "belong" to the parents. The kids might be able to help with decorating the nursery, child-proofing the house and even choosing names. The kids can begin to see the baby as a special new person in their lives, too.
Some children might be interested in going through outgrown toys and choosing some to give to the new baby, but it is important to be cautious with this. The children should not be led to think that everything they cared about now belongs to someone else. Don't push the children to give things away, but encourage them to be generous if they want to.
Children in a split-family situation may be especially sensitive to the idea that dad will now have a different family and won't need them any more after the new baby arrives. It will be very important for the adults—both biological parents and stepparents—to help the children realize that there is lots of love to go around. Dad should also be careful with his words and his actions. He needs to refer to all the children as "his" children, not just the children he lives with, and talk about the baby as the children's brother or sister. He should clarify how visitation will be handled after the baby arrives. If there will be changes, the children need to understand those changes.
Hopefully, the children will have the same amount of time with their dad after the baby arrives as before. Some of that time will be with a bigger family group, but it will also be good if dad can spend some time with each of the children alone now and then.
All of those things should reduce the sibling rivalry and increase the feeling of belonging for everyone in the complex family constellation.