Should George and Ann be justified or condemned for having cut off all relations with their father for so long because they believed he was guilty?
It is more complicated than Ann and George just believing that their father was guilty of the crime. Ann and George turn from their father out of a sense of shame. Their family was destroyed by the scandal, their parents separated, they had to move out of the neighborhood they loved. They were very angry at their father for ruining their lives. So I understand why they turned away from him.
In view of the fact that before the faulty airplane parts scandal which led to the jailing of both Joe and Steve, all these people were very close. They lived near each other, spent their lives growing up side by side and shared a similar understanding of morality. When Joe Keller turns and blames Steve Deever, exclusively, for the faulty airplane parts, Ann and George are shocked that their father would behave in such an unethical, criminal way.
Joe Keller is a strong personality, he is very convincing as the unknowing victim in the situation. He is released from prison, and once he is free, he makes sure that everyone, all the neighbors, his family, know that he feels pity for Steve, after all, he wasn't in the factory the day that Steve made the decision.
However, it is shortsighted of them to think that their father was capable of making such an important decision. The factory belonged to Joe Keller, he was the boss. When George finally goes to talk to his father, he learns the truth.
I think phrasing the question in terms of "justified" or "condemned" sets up the answer to be juridical (don't these terms remind you of some of the arguments of the Reformation?) In terms of what we know at the end of the play, it's clear that they should not have abandoned their father. There is no evidence that he was ever a "bad" father, and there was no real evidence that he did anything criminal. Perhaps, if he were a stronger man (and there is plenty of evidence presented that he was not) he could have stood up to Joe (clearly a very strong man); I don't think you abandon a man for weakness.
Part of their problem reflects the idealism of Chris in relationship to his father. Despite his failings, Joe realizes that family/relationships is/are essential even though they will never be perfect. Chris wants perfect; I think he should have read more Hawthone. This demand for perfection leads to his father's suicide and leaves Chris a broken man, at least temporarily, at the end of the play.
We all make mistakes, some bigger than others. It may be the responsility of the state to punish; I don't think it is the responsibility of the family to do the same. The New Testament tells us to hate the sin but to love the sinner. This may not always be possible, but given the ambiguity of Steve's situation, I think it would provide the answer for George and Ann.