Do you mean for your partners/parents that given born of your child and then you would another child that was given born?
In the sidebar, you can see an option that says, "Add brother/sister" in the "Personal" sidebar category. Click yours/your old family members with "Add brother/sister". With it, you will get a family member that says, "Sibling of (name)" from yours/your old family members.
P.S: Hope that helps for assistance, if I mistake it, I would try another chance to reply this answer. ~ Daniel