First of all, that's awesome!
But while you're encouraging your sister to develop critical thinking skills, do you question whether or not you are indoctrinating her into negative beliefs towards religion?
I know parents love to indoctrinate their children into their religion of choice (one of the many reasons I'm not on good terms with mine) but the opposite can happen, especially between friends/siblings where one person is unintentionally aggressively pushing against religion while the other has some topics they'd like to discuss but doesn't feel comfortable doing so.
Just food for thought, I'm sure you know your sister better than anyone else.