Tuber as an atheist you have a rich field to work from to involve your step mother in developing critical thinking skills. It is possible to take her on an interesting journey without appearing to be specifically critiquing Jehovahs witnesses per se.
For example, the Revelation verses are using a tried and tested device to set up oppositions between true and false, good and bad. This device is the portrayal of the other as luxury loving, effeminate (the woman), drunken, lying, barbarian (babylonian) and full of excesses to delineate oneself as pious and dutiful in the service of TRUTH.
Aeschylus' Persians is a good example of this. In this play the personas of the Persian Queen and Persians themselves including Xerxes are very similar to the woman, Babylon the great in Revelation.
When I use these methods I tend not to have goals in mind leaving it up to the person to make discoveries for themselves. It can be quite stimulating resulting in a journey of one's own.
btw I really like the way you are tackling your situation, questioning how deuteronomy fits with Revelation. As humans we love neatly fitting stories and are especially susceptible to being taught by this means. But Deuteronomy and Revelation do not fit neatly together as Jehovahs witnesses suggest.