Is it permissible for my fiancé's parents to see me without a hijab after the marriage contract, knowing that my fiancé has embraced Islam, but his parents are still Christian?

A woman is permitted to remove her hijab (definition) in front of her husband's parents because they are her mahrams (unmarriageable kin). There is no difference whether the mahrams are Muslims or disbelievers. Ibn Taymiyyah mentioned that if a disbeliever contracts a marriage that is forbidden in Islam, this establishes lineage and affinal kinship, and it renders the ascendants and descendants of each spouse forbidden to the other, by the consensus of scholars.