GTTM,
Next time she calls you an "apostate" tell her that you resent that term and that name-calling is juvenile and hurtful.
Tell her as you do not want her to talk to the elders about your private marital relationship and that no matter what, YOU will not talk to them. (As you correctly said, they have no legitimate authority over you.)
Finally, reassure her of your love for her and love of truth (the real truth).