Relationships in your home are strained anyway. You fear it will get worse. Buddy, it's getting worse already- the boy is going to be a full JW just like his mother, and the both of them will disrespect you further. The step-father aspect means nothing in the JW world, only in a court if she were to divorce you. You are the head of the household- PERIOD.
If she left you, she could take her son. But in your house, the JW's see you as the head. Use that headship or prepare for more of the same walking-all-over-you.
If you help the boy to learn the truth about Jehovah's Witnesses, maybe his not becoming one could help his mother see the truth about them. MAYBE, and maybe not. But trying to hold back leads to them usurping your headship and going around your back and making the boy into a JW and that gives his mother even more reasons to stay active the rest of her life.