two women represent two covenants.1 whose children are born in slavery is Hagar, she represents Mount Sinai covenant

The Example of Hagar and Sarah

Galatians 4 

²¹ Let me ask those of you who want to be subject to the Law: do you not hear what the Law says? ²² It says that Abraham had two sons, one by a slave woman, the other by a free woman. ²³ His son by the slave woman was born in the usual way, but his son by the free woman was born as a result of God's promise. ²⁴ These things can be understood as a figure: the two women represent two covenants. The one whose children are born in slavery is Hagar, and she represents the covenant made at Mount Sinai. ²⁵ Hagar, who stands for Mount Sinai in Arabia, is^[b] a figure of the present city of Jerusalem, in slavery with all its people. ²⁶ But the heavenly Jerusalem is free, and she is our mother. ²⁷ For the scripture says,

“Be happy, you childless woman!
    Shout and cry with joy, you who never felt the pains of childbirth!
For the woman who was deserted will have more children
    than the woman whose husband never left her.”

²⁸ Now, you, my friends, are God's children as a result of his promise, just as Isaac was. ²⁹ At that time the son who was born in the usual way persecuted the one who was born because of God's Spirit; and it is the same now. ³⁰ But what does the scripture say? It says, “Send the slave woman and her son away; for the son of the slave woman will not have a part of the father's property along with the son of the free woman.” ³¹ So then, my friends, we are not the children of a slave woman but of a free woman.