I can see why this type of thing can chafe. Personally, I come from a family tht proselytizes, but more in the form of "This is what we believe," then it is a mtter of living agfter that as a witness. NEVER would any of them or myself decide that it is up to US ti save someone. Afterall, it isn't us who saves people. This woman sounds like she has a wee bit of a control problem and it is being expressed through how she handles her (and your) religion. My guess is God doesn't really figure so largley, rather it is her aim to "earn brownie points in heaven" by claiming you as a trophy. Sorry for the harsh tone, but I have also had to deal with folks like that, and no, you can't go on ignoring her forever. I suggest a serious talk with your husband, even if it is more of a "fact finding" mission to see how he thinks you can get her to stop calling. I think that is how it should be worded, because if you ask "What should do?" He'll say something convenient and there you'll be in square one.
BTW some of the funniest and best discussions I have had have been with a sunni muslim friend, when we would sit at lunch in the psychology institute and try to convert each other. Usually ending in a lot of laughs. Your sister in law needs to realize if you don't want to talk about it, then she needs to shut up and start living the life herself. Huggs dear and good luck!