Am I sinful before God regarding my husband's tyrannical mother, whom I insulted by saying her words lacked manners, and then I apologized to her, but she refused to forgive me? And is it a sin if my husband does not speak to me until she is appeased, which is impossible due to her humiliation of me?

If you retaliate in kind without transgressing, there is nothing upon you. Neither your husband's mother nor your husband has the right to abandon you. If you erred, you should have been forgiven after your apology, for it is not the habit of the noble to reject one who apologizes. Your husband should not be hard on you and should strive for reconciliation. You, on your part, should appease his mother seeking Allah's reward and out of kindness to your husband.