The Way Back implies that Blake has been getting forged tapes from his brother and sister, who actually are dead.
Wouldn't the lie, if any, go in the other direction? At that stage, Blake is a convicted and supposedly rehabilitated rebel and traitor. Wouldn't The Powers that Be want him to feel a constant sense of guilt for getting his family killed (whether or not this had actually happened) instead of wanting him to think that his family were still alive off-world? In the latter case, they *could* serve as hostages for his continued good behavior, but in the former, he would be an awe-inspiring example of the high cost of rebellion and therefore an effective deterrent.
-(Y)