First, hugs to you and what you are going through. I can't imagine the added stresses are making getting pregnant any easier either.
IMO, since she came clean about spreading these rumors she seems to believe, has anyone asked her genuinely why she believes you are lying? And I don't mean yell at her in a confrontation way. But truly sitting down and opening up and saying, "this whole situation is really hurtful to me, can you tell me why you feel this way" kinda thing.
I highly doubt your husband feels you are lying, he seems to be on your side. You don't say how close DH and his mom are, but her lies could end up tearing the whole family apart, especially when you do have a positive test. Maybe she doesn't believe there are such things as fertility problems.
All in all, I don't think cutting your ties altogether is the answer, but I think you and DH need to give the situation a break and continue with your own lives without including everyone else. And why is he discussing your fertility concerns at work with his father and brother? Some things should stay private.
Best of luck