How do i convince my sister from telling my mom im gay?

So my family is hard core christian. (no offense, theist) my 13-year-old sister found out im gay. (im fourteen, girl) she is going to tell my mom when she gets back from this bible thing she is doing (in three days) i have tried everything, but she wont budge. I keep trying to help her understamd that this is not a choice or a disease, but she insists that she tell her. What can i say!!!?