How about showing your mum a quote from the old Truth book:
Since we do not want our worship to be in vain, it is important for each one of us to examine his religion. We need to examine, not only what we personally believe, but also what is taught by any religious organization with which we may be associated. Are its teachings in full harmony with God's Word, or are they based on the traditions of men? If we are lovers of the truth, there is nothing to fear from such an examination? (The Truth that Leads to Eternal Life, 1968, p 13)
Actually does anyone have a Truth book they can scan: I'd love a scan of that page for my own parents.