I'm sure no matter what, he wont tell my parents or the elders.
There have been quite a few threads on here where people have been ratted out by Dubs they thought they could trust.
Tell your brother nothing until after you have finished your degree in a related branch of psychology.
Ask him a couple of well selected questions in such a way that he feels he must find an answer, but do that without telling him anything new. Choose something out of current literature and add nothing but critical thinking.
Any question must undermine selection in 1919 to be god's sole channel as that is the primary doctrine that holds them all captive. It is the blood supply that keeps belief in their non-scriptural, unscientific, historically incorrect, dishonest, doctrines alive.