Thank you for the question on Seth, “Did God appoint Seth to be a replacement for his older brother Abel?”
God declared to Adam and Eve to fill the earth so that the humans would care for the Garden and the world’s creature. Genesis 1:28 says this, “God blessed them and said to them, “Be fruitful and increase in number; fill the earth and subdue it. Rule over the fish in the sea and the birds in the sky and over every living creature that moves on the ground” (A searchable online Bible in over 150 versions and 50 languages). Bible citations are in the New International Version.
Cain murdered his brother Abel (Gen. 4:8). Cain and his family left the Garden and moved eastward. In their grief for the lost of their two sons, one being dead and the other moved away, God blessed Adam and Eve with another son, Seth, and other sons and daughters (Gen. 5:3–5).
Thus one can say that Seth is the third son of Adam and Eve. From a theological viewpoint, one can say that Seth is the next appointed son of Adam and Eve to carry on the lineage of a godly line. The Scripture is silent as to whether Abel was married or whether he had children.
SUMMARY: Seth is the third son of Adam and Eve. It is through Seth that the godly line of Adam and Eve is preserved to Abraham and then to Jesus.
For more perspectives:
https://www.quora.com/Did-God-appoint-Seth-to-be-a-replacement-for-his-older-brother-Abel