If we Gentiles are also descendants of Abraham, then from whose mother: Hagar, Sarah or Keturah? Is it from Keturah to whom she bore Abraham six legitimate sons after Abraham married her after Sarah died? Am I correct to say that Muslims are descendants of Ishmael who later intermarry with the descendants of Esau, first born son of Isaac. Israelites descendancy come from Jacob (Israel) also son of Isaac, who is the son of Abraham with Sarah. Abraham is indeed the father of three great nations - Muslims, Jews and Christians!

You are confusing physical descendants and spiritual descendants. Physically, Abraham is the father of many nations, Israel, Edom, the Arab nations, and several others. However, he is not physically the father of all nations.

However, because of his great faith, Abraham is the father (forerunner) of those who have faith; that is, Christians. "And he [Abraham] received the sign of circumcision, a seal of the righteousness of the faith which he had while still uncircumcised, that he might be the father of all those who believe, though they are uncircumcised, that righteousness might be imputed to them also, and the father of circumcision to those who not only are of the circumcision, but who also walk in the steps of the faith which our father Abraham had while still uncircumcised" (Romans 4:11-12).

"Therefore know that only those who are of faith are sons of Abraham. And the Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel to Abraham beforehand, saying, "In you all the nations shall be blessed." So then those who are of faith are blessed with believing Abraham" (Galatians 3:7-9).

The only relationship between Abraham and Muslims is that some Muslims are physical descendants of Abraham.

See also:

Questions and Answers regarding the Old Testament
Questions and Answers regarding the Church