Sit them both down together and let them know they are now family. Although you do not expect them to get along all the time, or even love each other at first, let them know they are siblings and they protect each other. Let them know how grateful they should be that they now have a brother/sister. Start conversations with them, music, friends, school, things that kids are into nowadays. They may find that they have a lot of things in common. Right now they see the other as a predator. You were your son's mother and now you get a daughter, he doesn't want to share. Same with your SD. Sit and play games as a family. When they realize that the other isn't trying to steal the parent away from them, things will calm down.
at 11:47 AM on Jun. 17, 2011