If you want to add half brothers and sisters, then you have to add the step-parents first. When you add the children you will be asked which parent you want to assign the children to.

Example:

You have a half-brother from your father's former marriage. In order to add this brother to your father and his former wife you will first have to your father's ex-wife. Then when you add your half brother a menu will appear and let you choose which of your father's marriages the child came from.