It rarely does more than either make you angrier or gets you converted too. Wait until you are educated enough to take this step. When you are fully prepared you can then attend their meetings etc and discuss this with him rationally. Don't be jealous of the cult. The tendency is to think that love of home and family is enough incentive for them to leave.
Mind control is stronger than family ties.
Do offer love, support and understanding to the cult victim during the time in the cult, coming out of the cult and during recovery. In any illness there is a period of convalescence.