Thank you for asking such a thoughtful and interesting question. Dynamically, you and your husband share a familiar role in your respective families. It sounds like the narcissism has been a powerful and difficult feature each of you have had to deal with each of your mothers.
I recommend that the two of you discuss your vulnerability and needs with each other on a regular basis — not just in crisis time. (Couple therapy may be found at the ‘Find Help’ tab at the top of this page if you feel you need help in doing this.) You want to continually join forces and share the difficulty in doing that with each other. Your families are the common enemy in this regard. It is difficult to forgive (and some research shows it is best not to do so too quickly) when the person has an abusive and narcissistic feature to them.
I’d also recommend Alice Miller’s classic book: Drama Of The Gifted Child. It discusses in depth the issues of growing up with a narcissistic mother.
Finally, I’d begin to pull back significantly in the frequency and intensity of contact with both families. Your future is together with your husband, not staying in orbit around the chronically self-absorbed.