It seems to me that you should see your step mother as an allie and try to quietloy without yoru father knowing develop a common strategy to draw your father and your brother away from the cult. I am sure she would be very happy if you approached her in this way as I am sure she is just as frustrated with your father's membership of the cult as you are. If you both show him unconditional love in contrast to the conditional love he displays, it might get him thinking. I am a firecely opposing unbelieving mate so I know how difficult it is even though communication is allowed.
As others have said - you should not treat this latest statement that he needs to cut you off as final - he will be back - you are his daughter and he loves you, cult indoctrination notwithstanding. Good luck