Will the wife be sinful if she does not visit her husband's oppressive mother who abuses and insults her and her family, knowing that she goes to visit her when her husband asks her to, and will the anger of the husband's mother towards her son because of his wife befall him?

You are not obligated to visit your husband's mother, especially if she is aggressive and unjust. However, your going with your husband to visit her is a good deed for which you will be rewarded, and it will improve the relationship between you two. If the husband's mother is angry with her son because of you without a just cause, then no consequences arise from it. Nevertheless, the husband should strive to appease her as much as possible.