Just keep talking and keep negotiating and making sure you know exactly where everyone else stands. She's probably terrified that her husband will want you more (someone has to chime in here about all that NRE business) and for a time he might. But, if you're committed to being with both of them, your actions regarding that commitment should speak for themselves and hopefully things will mellow.
There are a lot of things we do now very comfortably that I never thought would come about in my MFM triad, mostly because I didn't think T would get to the point of relaxing enough to know that I'm not going to run off with E. But, E would likely say that he spent a lot of the first few months of us being together sleeping alone and feeling a little less than equal until the balance came.
I think the secret is to communicate, communicate, communicate and make your eventual intentions known through statements like, "Ultimately, I'd like it if we could all (whatever)." That's what E did, T noticed and we got to a point where we all started moving forward, together.