If my future BIL got me this I would tell him eff no (thanks but no thanks, depending on sobriety). If my future FIL got me this I would definitely have a talk with him about it, and my concerns, but would respect his wishes if it was really important. Fortunately for me, my in-laws are no more religious than me so that sort of thing isn't an issue.
Wacky, scale of 1-7, how intimidated are you by her family? You are going to have to deal with them the rest of your life. If you let yourself get pushed around, it will only continue and become more expected.