In order for that to stop, all parties would have to be willing to do what it takes to stop it. Part of that would be for her sisters to stop harassing her about you, which isn't likely.
Anything that you might do in hopes of making it better for both of you would probably make the situation worse. You can only be there to listen and support your wife.
There is something that your wife can do. She can tell her sisters that who she chooses to be with is her choice and if they want their relationships with her to continue, they have to keep their comments to themselves. If she's with them and they start talking negatively about you, she can simply get in the car and go home. Visit over! It might take doing that a few times, but if they really are close, it's likely that they'll stop in order to save their relationships with her.
Something else she could do is stop defending you and act like their comments don't bother her. If they don't get a defensive reaction, they might stop.