How can one deal with a mother-in-law's frequent and prolonged visits to the marital home, and is a wife entitled to express her discomfort to her husband without sin?

There is no sin upon you in informing your husband of your distress regarding his mother's prolonged stays, for you have the right to an independent dwelling. However, if the house is spacious and her presence does not cause harm or expose your private matters, then you do not have the right to prevent her. We advise you to discuss the matter with your husband gently and wisely, explaining that you do not object to her visiting for appropriate durations, and that he should honor his mother without causing you harm.