This isn't right dude. Abraham did not sin by having a child from Hagar. As a matter of fact Sarah asked him to do so but this isn't the case here. The thing is that during those days, having sex with your female slave was not considred a sin. All of this can be verified.
Even if Abraham sinned, he is still his son and GOD didn't say he isn't his son.As a matter of fact, this bastard son was blessed.
We have to understand that those people have different values, they didn't live in the 20th century. During those days they had different rules.
Let me tell you a litle bit about the rules they followed:
Is a son is born to a slave, he will have the right to inherit every thing if no son came from the legit wife, however, if a son is born to the wife, then they will share everything.
All of this could be verfied through the archeological discoveries made in Iraq.