Would Abraham have committed a serious sin if he had refused to sacrifice Isaac?

No. But he would not have been chosen as the man of faith God would use to start the Jewish faith. You can only sin if you go against a moral law of God. If God asks you to do something over and above that, you do have the right to say, I am sorry, I cannot do that. Then God makes his decision as to whether you did that deliberately or not; he will be able to judge. But you can say no. God would just go get another man, somebody who would fill the requirements of the kind of man God wants to start this religion.