If you are going to foster short term children, then you would prepare your family to welcome new children and say good-bye to those leaving. It is possible with your guidance for your children to accept this and understand their family identity as a a short-term foster family.
If you are going to foster long term and possibly forever children then you would have to prepare your children for that way of life. Will your foster children have the same rights as your children? Will a trip to Disneyland include all your children? These things have to be thought through before your family actually accepts foster children. How will you handle differences between natural children and foster children? Each will pull you, each will try to get you to take "sides". Teaching all the children how to problem solve will bring peace to your family. Remembering that blame doesn't matter, who started it doesn't matter, only that there is discord in the family, and that it needs to be healed.