PolyAus, consider this: do you think you might be projecting your own insecurities onto your metamour? You say you don't subscribe to a hierarchical structure, but perhaps there is still a fear in you about being "second" anyway. Maybe it is you who compares yourself to her and enters situations with a "chip" on your shoulder, so to speak. Not an angry chip, necessarily, but an insecure one. It sounds like you feel as if there is some sort of mark against you for being younger, less experienced, not a parent, etc., and since that's an uncomfortable place to be, maybe you turned it around in your mind to assume that she is jealous or resentful toward you because of it. She may not be that at all. And perhaps there is some apprehension you feel about moving in together, but you're putting all the "blame" on his wife, to say she will be the one who will make it difficult for you, but you might be overlooking some fears and nervousness you have in yourself about it.
Just something to think about.