If she points to the Bible as her ultimate authority, then just show her 1 Corinthians 7:13 - as The Fall Guy suggested. Otherwise i can only extend my deepest sympathies - i am only nineteen, so i do not have that much experience with such matters. I do however know how it is to loose ones family, and even though they are still pimi, it breaks my heart almost daily not being able to have a "normal" relationship with them. Keep fighting, as long as possible.
Best regards
WorldCitizen