Has your husband told his sister, in no uncertain terms, that her lashing out is only doing more harm than good? Clearly, she's hurt, after all her husband cheated and you are the other woman, however, she's going to need some couples counseling to move past this. Harassment is Serious. Eventually, if you and your husband don't fan the flames as active participants in this, she'll burn herself out, quite literally.

You made a mistake, owned it, and as husband and wife and child, are trying to move past it, and live your lives. Eventually, others will grow tired of the melodrama. If others confront, simply sigh, say something like oh no, not again...and end the conversation.