Is the questioner sinful if she tells her husband's sister that her mother-in-law does not wear the hijab in front of her ex-husband, and what is the ruling on the mother-in-law boycotting her as a result?

There is no sin upon you for what you did, mentioning your mother-in-law in a way you dislike, with the aim of seeking help from someone to reprimand her. This is permissible and is an exception to the forbidden backbiting if the intention is to remove an evil. As long as you greet her when you meet, you have not fallen into the forbidden abandonment (hijr) according to Islamic law, even if she refrains from greeting you or entering your home. The forbidden abandonment is the ترك السلام upon meeting due to animosity and hatred, and the abandonment ceases simply by greeting and returning the greeting. The husband's mother is not among the kin whose ties must be maintained, unless she is also a relative of yours.