By faith Sarah was able to become a mother even when she was too old to have children. God had promised this would happen, and Sarah believed that he was faithful.
Your relative Elizabeth is about to have a child even though she is old. People thought she couldn’t have children, but she has been pregnant now for six months.
In other words, God’s children aren’t all those who are physically descended from Abraham. Rather, they’re the children God promised to him. Those are the ones who are considered to be Abraham’s children.