How do you tell your mom-in-law that you're not a believer and follower of superstitions as regards parenting?

For me, I would just smile it off or if I have the temperament for it (usually I don't) I'd humour her and ask her for a further explanation for the superstition. I don't have to follow anything, just politely listen and usually that's enough. I realise that if I argue or try to reason with older folk who are superstitious, it never ends.