She continues to feel threatened, either because she never really wanted an open relationship, or because she wanted to get married and he didn't, or there are other issues in their relationship that is swept under the carpet. So they come up at every turn. Ask him to take her seriously. The things she says are not about you, they are about them, you are just the trigger. They have stuff that need to be discussed. He is a sloppy hinge for sweeping their issues under the carpet. One day you will be the recipient of the same sloppy treatment, and you will discover your resentment popping up where it doesn't belong. The short term solution would be to keep some distance to her, ask him to respect that. The long term solution would be counselling for them.