Maintain a relationship with her. So you at least get a chance to find out the source of your confusion with it. Why do you feel in-law as a threat? Talking, you will achieve more than conflicting. Open dialogue is always better to violent disputes and unexpected encounters.
Be patient with her. In the end, it is in fact older than you. And perhaps her experience will be useful to you, as well as her advice that she gives.
Do not cling to her every word, a deaf ear to its critical or sarcastic remarks. So you keep peace in the family and your own peace of mind. Engaging with it in verbal disputes and squabbles, you will only strengthen its negative attitude towards you. Every meeting with her spend as if between you never had negative emotions.
Ask for forgiveness from her mother in law. Yes, strange as it may sound. Leave time for your pride and tell her these words. It certainly feels moral satisfaction. But, perhaps, you will feel better. And that is exactly like your husband. After all, he loves not only you, but also his mother.
When the sister-in-law blames it clearly does not want to understand that in the resulting situation there and its wines. The young woman could not or did not want to go the way of the gradual establishment of relations, and would prefer an open clash. Try to treat it, if not as to his mother, then as a loved one. No doubt she will appreciate it with time.