First, I want to second RedPepper's hat's off to you. You are an amazing spirit, and you must love your husband very, very much indeed. If his love is equal to yours, y'all will undoubtedly find your way through this vale of tears.
It's wonderful that you and he have been working on getting your marriage back on solid ground; IMO, that's exactly what must be done for to clear the way for any future happiness and stability in your relationship, regardless of what else happens. Heal this relationship first, then go from there. Adding additonal relationships to a troubled one is a recipe for disaster, again IMO.
And don't feel like you have to rush through the healing process. You've gone through a terrible trauma, created by your nearest and dearest. Your trust has been undermined, your security in those relationships destroyed. It's an earthshattering experience; I know. It's okay to go slow, as slow as you need to, and to do the things you need to do to heal.
What do YOU need for your healing? What do you need to be healthy and whole in your relationship to him, to her, and to the two of them?
When your heart aches, or when you find yourself yelling and crying, take the time right then to ask your deepest self why? What's causing this? Look at the pain; it's coming from the places that need healing. Spend some time looking deep into your heart and figure out what you need and want. Then ask him/them for it.
Good luck to you, SisterWoman, and godspeed.