Hi, Zoewrex! (interesting screen name, btw)
I'm with Valis, too. Just tell your dad and step mom that you appreciate the positive changes their new religion has made in their lives, but that after reading CofC (which you said you're buying, right?), you feel very concerned about what they've gotten into. Nonetheless, if they're happy, fine, but you, too, are happy with your religion, therefore you think it's best for all concerned if religion is not discussed whenever you're together, as it will only lead to hard feelings, etc., etc. (in that same vein).
This way, you MAY make them curious about Crisis of Conscience and what an ex-Governing Body member has to say, but at least you and your child won't be the object of their proselytizing when you're trying to DEstress!
Welcome to the Board!
outnfree