I don't know what to say about your Dad's response - I wouldn't want to be in his situation - keeping important good family news from one daughter on the whim of the other daughter; it's not nice to be played off like that.
I can't understand the reasoning behind your sis not wanting you to know - how in hell could that possibly hurt... ??
We all know this cult makes people behave in strange ways, maybe there is some comfort in that - your sis is acting out of character ?? You've been away for a long time now, and she will have had much more propoganda bombarded at her day and night to reinforce that she has done the correct thing (in their twisted view)
Sorry you found it in a ham fisted way too.
Thinking of ya, and I'm sure your B/F is giving you all the support you need right now - he sounds like a really nice chap !!