No, it is not a sin to be a white supremist or join the Aryan Nation, or the KKK. The Holy Bible never ever condemns racism, because God is a racist who has chosen one race to be special above all races in the Old Testament.
Jesus Christ clearly showed Himself to be a racist against Samaritans in the New Testament and perpetuated racist stereotypes against Samaritans because racism is NOT a sin. The Bible never ever condemns racism.
So every American has not only the right to join the Aryan Nation or the KKK or any other white supremicist group. joining these groups is not sin.
What would Jesus do? Jesus was a racist-supremicist who showed Jews during His ministry on earth that Jews are racially superior to the Samaritans. Jesus Christ made perpetuating racist stereotypes against Samaritans and other races as a part of His earlthly ministry, thereby proving to all believers that it is not a sin to be a racist.
Jesus Christ never attacked racism, which was rampant in His society, because racism is never sin in the Holy Bible, and racism will never be sin until the Second Coming of Christ.
All who attack racism as sin are attacking Jesus Christ and the Gospel of Jesus Christ, which perpetuates racism.
You have to decide whether you are a follower of Christ and the Gospel, no matter how offensive it is, or you are going to create your own sanitized religion. But you have to suffer for your choices, because Jesus Christ is God, and Jesus Christ is always right.