My mother-in-law asked me for the key.

..... If you bought the house yourself, you don't have to give it to her, or you can tell her the main idea. Anyway, she prefers your husband's brother to live with her husband's brother, right? You have no reason to create opportunities. Change the key. At that time, she will ask, and she will say that she lost the key and change it now, but I think it is better for your husband to tell your mother-in-law himself.