He was too old to have children, and Sarah could not have children. It was by faith that Abraham was made able to become a father, because he trusted God to do what he had promised.
Now Elizabeth, your relative, is also pregnant with a son though she is very old. Everyone thought she could not have a baby, but she has been pregnant for six months.
This means that not all of Abraham’s descendants are God’s true children. Abraham’s true children are those who become God’s children because of the promise God made to Abraham.
Abraham’s son from the slave woman was born in the normal human way. But the son from the free woman was born because of the promise God made to Abraham.