I would totally cut them off for that. Do not, under any circumstances, rejoin the witlesses or try to get reinstated just for them. The parents are elderly, and will probably soon be in dire straits and need your help. At which point, just refuse--you are not responsible for helping out parents, and if they are mean-spirited, you are under no moral obligation to honor them.
Once they run into financial difficulty or health problems where they need your assistance, and you refuse, they will see the effects of their actions. If they continue hounding you, you could tell them that they are going to have to rely on Jehovah for help. Remember, the prodigal son had to suffer like that before he came to his senses.
Or, you could offer the help, but under the strict condition that no attempt will be made to get you to rejoin the cancer. They could in fact be exposed to how their religion is not the truth. If it gets to that, they will not have much choice. Either way, you should under no circumstances go back into the cancer--or else, when you get old, it might be your turn.